feat: ESM support

[stipsan]

Can be tested using npm i get-it@esm.
There's also a @sanity/client@esm that lets you try it out.

Changelog:

• Prepares Semantic Release, it's blocked with a --dry-run flag for until it's run on main and we've verified it's good to go.
• Moved to pkg.files instead of .npmignore.
• Added GitHub Actions CI workflow that
  • Builds, lints and tests in Node LTS for ubuntu, mac and windows.
  • Runs mocha in node v12 and v18 on ubuntu.
  • Runs a new test suite that checks pkg.exports and verify they work both when imported to a .cjs and a .mjs file. Uses Node 18's new built-in test runner to ensure the testing env doesn't handle pkg.exports differently than native node ESM.
  • Adds a new testing suite that uses Node 18 and -C browser to test pkg exports when the browser conditional is active, just to catch easy mistakes like typos.
  • Adds a Deno test that runs in Node compatibilty mode to test the exports.
• Adds pkg.exports for browser and deno that's native ESM.
• Adds backwards compatible pkg.exports (that's why there's so many of them!) to ensure that by adding pkg.exports we're not creating a breaking change to existing node environments that works fine with get-it as it is today.
• Compiles the ESM using esbuild in --platform=browser mode to apply pkg.browser mappings to ensure minimal changes to existing code is required, and greatly reducing the risk of regression bugs.
• It's using type: commonjs instead of type: module for the same reason, to avoid introducing breaking changes in systems that currently work with get-it but would break if we started moving around files. By using type: commonjs and .mjs file extensions we ensure every flavor of Node.js keep working.

Why both deno and browser conditions? Because deno doesn't support browser when it's in compat mode: https://client-runtimes-dashboard.sanity.build/#:~:text=deno%2C%20import%2C%20node%2C%20node.deno%2C%20node.import

The list over supported ESM browser/deno/worker mode exports is based on looking at how get-it is commonly imported:

• https://cs.github.com/?scopeName=All+repos&scope=&q=get-it%2Flib%2Futil
• https://cs.github.com/?scopeName=All+repos&scope=&q=get-it%2Flib-node%2Futil
• https://cs.github.com/?scopeName=All+repos&scope=&q=get-it%2Fmiddleware
• https://cs.github.com/?scopeName=All+repos&scope=&q=get-it%2Flib%2Fmiddleware
• https://cs.github.com/?scopeName=All+repos&scope=&q=get-it%2Flib-node%2Fmiddleware

[rexxars]

Wow. That's a huge amount of work - thank you!
Let's make sure to release this as a major version just in case?

[stipsan]

Yeah let's err on the side of caution ⚠️