LibJS: Implement the Uint8Array to/from base64 proposal

[trflynn89]

test262 diff (we pass all of the Uint8Array tests):

Diff Tests:
    +56 ✅    -56 ❌

Diff Tests:
    test/built-ins/Uint8Array/fromBase64/alphabet.js                         ❌ -> ✅
    test/built-ins/Uint8Array/fromBase64/descriptor.js                       ❌ -> ✅
    test/built-ins/Uint8Array/fromBase64/ignores-receiver.js                 ❌ -> ✅
    test/built-ins/Uint8Array/fromBase64/illegal-characters.js               ❌ -> ✅
    test/built-ins/Uint8Array/fromBase64/last-chunk-handling.js              ❌ -> ✅
    test/built-ins/Uint8Array/fromBase64/length.js                           ❌ -> ✅
    test/built-ins/Uint8Array/fromBase64/name.js                             ❌ -> ✅
    test/built-ins/Uint8Array/fromBase64/nonconstructor.js                   ❌ -> ✅
    test/built-ins/Uint8Array/fromBase64/option-coercion.js                  ❌ -> ✅
    test/built-ins/Uint8Array/fromBase64/results.js                          ❌ -> ✅
    test/built-ins/Uint8Array/fromBase64/whitespace.js                       ❌ -> ✅
    test/built-ins/Uint8Array/fromHex/descriptor.js                          ❌ -> ✅
    test/built-ins/Uint8Array/fromHex/ignores-receiver.js                    ❌ -> ✅
    test/built-ins/Uint8Array/fromHex/illegal-characters.js                  ❌ -> ✅
    test/built-ins/Uint8Array/fromHex/length.js                              ❌ -> ✅
    test/built-ins/Uint8Array/fromHex/name.js                                ❌ -> ✅
    test/built-ins/Uint8Array/fromHex/nonconstructor.js                      ❌ -> ✅
    test/built-ins/Uint8Array/fromHex/odd-length-input.js                    ❌ -> ✅
    test/built-ins/Uint8Array/fromHex/results.js                             ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/alphabet.js            ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/descriptor.js          ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/detached-buffer.js     ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/illegal-characters.js  ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/last-chunk-handling.js ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/length.js              ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/name.js                ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/nonconstructor.js      ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/option-coercion.js     ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/results.js             ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/subarray.js            ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/target-size.js         ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/whitespace.js          ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromBase64/writes-up-to-error.js  ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromHex/descriptor.js             ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromHex/illegal-characters.js     ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromHex/length.js                 ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromHex/name.js                   ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromHex/nonconstructor.js         ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromHex/results.js                ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromHex/subarray.js               ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromHex/target-size.js            ❌ -> ✅
    test/built-ins/Uint8Array/prototype/setFromHex/writes-up-to-error.js     ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toBase64/alphabet.js                 ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toBase64/descriptor.js               ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toBase64/detached-buffer.js          ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toBase64/length.js                   ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toBase64/name.js                     ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toBase64/nonconstructor.js           ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toBase64/omit-padding.js             ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toBase64/option-coercion.js          ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toBase64/results.js                  ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toHex/descriptor.js                  ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toHex/length.js                      ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toHex/name.js                        ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toHex/nonconstructor.js              ❌ -> ✅
    test/built-ins/Uint8Array/prototype/toHex/results.js                     ❌ -> ✅

[awesomekling]

Some callers (LibJS) will want to control the size of the input buffer, to decode up to a maximum length.

Did we mean "output buffer" here?

[trflynn89]

I actually meant "the buffer supplied as an input to the function" but I see how that is not at all clear :)