Validate unique dictionary members (including inheritance)

[foolip]

https://heycam.github.io/webidl/#idl-dictionaries says:

The identifier of a dictionary member must not be the same as that of another dictionary member defined on the dictionary or on that dictionary’s inherited dictionaries.

I tried if webidl2.js would catch such errors but it seems like not:

const { parse, validate } = require("webidl2");
const ast = parse(`dictionary A { any m; }; dictionary B : A { object m; };`);
const validations = validate(ast); // returns []

[foolip]

It actually looks like duplicating the member name on a single dictionary is also not validated for.

[foolip]

I wrote up a quick check for this locally (in webref) but to my surprise there doesn't seem to be any duplicate member names currently.

[saschanaz]

Yeah, the current name check only applies for parent types, not members. Something to be done later.

[saschanaz]

I strongly feel that the implementation here will share the core logic with #584, but not something to worry about for now.