add index signature to ArrayLike

[trusktr]

• I've read the contributing guidelines

Without this change, I get a type error in TypeScript and VS Code (not in AssemblyScript).

The error in TypeScript is:

Element implicitly has an 'any' type because expression of type 'number' can't be used to index type 'ArrayLike<N>'.
  No index signature with a parameter of type 'number' was found on type 'ArrayLike<N>'. ts(7053)

[trusktr]

Without this change, a workaround is to place a // @ts-ignore comment above the offending line, just to satisfy TS.

[dcodeIO]

Seems OK to uncomment, yeah.

Background: The isArrayLike builtin currently checks that a .length property and an index signature is present, it just doesn't guarantee any types like T. As such I don't remember why exactly the index signature is commented out, but I'd guess that this is left over from an earlier version that didn't guarantee that an index signature is present.

[dcodeIO]

It would be good to pick one and stick with the format because the NOTICE file is also parsed to generate contributors in package.json for example.

[MaxGraey]

At the moment ArrayLike is just a stub to build-ins (like isArrayLike) and no concrete class implements this interface, so these changes doesn't make sense yet.