AST source locations for named mapping keys.

[ekpyron]

Following up on #13384: we missed the fact that the key names in that PR do not carry source location information, which will be an issue for tooling, which relies on everything in the AST being annotated with source fields.

It's not particularly nice, but the most straightforward solution would be to copy the behaviour for the names of VariableDeclarations (those have a nameLocation field besides the name field, i.e. we could just add keyNameLocation and valueNameLocation).

Once we accumulate a worthwhile set of breaking AST changes, we can generally transition parsing of the mapping keys and values themselves to parsing as VariableDeclarations - that'd simplify this, but we can't do it right away without breaking the AST, which we promised to avoid, when possible.

@zemse if you want to add those, let us know - although we'd need a PR rather soon (within the next week or so), since we're planning to release soon and this should be fixed beforehand. So if you don't have time, let us know as well ideally, then we can take care of this.