implement RegExp Match Indices proposal

[targos]

Refs: https://tc39.es/proposal-regexp-match-indices/

> /(?<x>a)(?<y>b)/.exec('ab')
['ab', 'a', 'b']
> /(?<x>a)(?<y>b)/.exec('ab').indices
[[0, 2], [0, 1], [1, 2]]
> /(?<x>a)(?<y>b)/.exec('ab').indices.groups
{ x: [0, 1], y: [1, 2] }

[targos]

The diff is easier to see with ?w=1

[targos]

Without this, the access to the values in groupNames will be off by one in MakeIndicesArray

[targos]

e can be >= Input.length

[targos]

/cc @rbuckton. I think I found two small bugs in the spec text.

[rbuckton]

@targos can you file issues for the bugs on https://github.com/tc39/proposal-regexp-match-indices/issues?

[targos]

@rbuckton done!