`RegExpExecArray` should have explicit always-defined properties

[DanielRosenwasser]

[L]ooking a little closely at the ECMAScript spec, I do think that there is a mismatch between what we have and what gets created in the runtime. RegExpBuiltinExec says that the following are created unconditionally:

• an index property
• an input property
• a property at index 0
• a groups property that is always set, but may be undefined

So RegExpExecArray should look something kind of like

// src/lib/es5.d.ts

interface RegExpExecArray extends Array<string> {
    // always present
    index: number;

    // always present
    input: string;

    // always present - helpful for '--noUncheckedIndexedAccess'
    0: string;
}

// src/lib/es2018.regexp.d.ts

interface RegExpExecArray extends Array<string> {
    // always present - helpful for '--exactOptionalPropertyTypes'
    groups: { [key: string]: string } | undefined;
}

Derived from content posted by Daniel Rosenwasser (@DanielRosenwasser) in #50211 (comment)

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In additionl to the above, it seems like RegExpMatchArray/RegExpExecArray are sometimes both returned from functions that undergo the same internal code paths. So we should probably ensure that one is compatible with the other, and have a test that ensures as much.

[DanielRosenwasser]

Relevantish because DetachHead sent #49682

[DanielRosenwasser]

Because people will come back to this issue, here's an example of where RegExpExecArray and RegExpMatchArray really should differ.

> "blahfoo".match(/(bar)?/g)
< (8) ['', '', '', '', '', '', '', '']

> /(bar)?/g.exec("blahfoo")
< (2) ['', undefined, index: 0, input: 'blahfoo', groups: undefined]

[TomasHubelbauer]

In your code sample I believe the last line should be this:

< (2) ['', { undefined, index: 0, input: 'blahfoo', groups: undefined }]

Edit: This is wrong :-)

[fatcerberus]

Tomáš Hübelbauer (@TomasHubelbauer) No, he was right:

[graphemecluster]

Actually there shouldn’t be anything like RegExpMatchArray. Because the spec states that String.prototype.match either returns either what RegExp.prototype.exec returns, or a string array with at least a single item, we should change the return type of .match to [string, ...string[]] | RegExpExecArray | null. And, please consider reverting #49682 since it breaks a lot of existing codes (#50633).

[DetachHead]

i think the solution is to add the 0: string to RegExpExecArray as well instead of reverting #49682. i'm happy to make a PR for this