[API] Expose globals declared by a source file

With TypeScript 6.x, TypeDoc supports documenting global files by documenting the globals defined in that file.

This was done somewhat hackily with:

```ts
result = checker.getSymbolsInScope(node, ts.SymbolFlags.ModuleMember)
.filter((s) =>
s
.getDeclarations()
?.some((d) => d.getSourceFile() === sourceFile)
);
```

@jakebailey told me on Discord that there is `SourceFile.Locals`, which would be a better way to get this information, but this isn't exposed in the JS API, so I'd like to request that!

Ref: https://github.com/TypeStrong/typedoc/issues/3098