Rollup merge of #69194 - Centril:assoc-extern-fuse, r=petrochenkov

Rollup merge of #69194 - Centril:assoc-extern-fuse, r=petrochenkov

parse: fuse associated and extern items up to defaultness

Language changes:

- The grammar of extern `type` aliases is unified with associated ones, and becomes:
  ```rust
  TypeItem = "type" ident generics {":" bounds}? where_clause {"=" type}? ";" ;
  ```

  Semantic restrictions (`ast_validation`) are added to forbid any parameters in `generics`, any bounds in `bounds`, and any predicates in `where_clause`, as well as the presence of a type expression (`= u8`).

  (Work still remains to fuse this with free `type` aliases, but this can be done later.)

- The grammar of constants and static items (free, associated, and extern) now permits the absence of an expression, and becomes:

  ```rust
  GlobalItem = {"const" {ident | "_"} | "static" "mut"? ident} {"=" expr}? ";" ;
  ```

  - A semantic restriction is added to enforce the presence of the expression (the body).
  - A semantic restriction is added to reject `const _` in associated contexts.

Together, these changes allow us to fuse the grammar of associated items and extern items up to `default`ness which is the main goal of the PR.

-----------------------

We are now very close to fully fusing the entirely of item parsing and their ASTs. To progress further, we must make a decision: should we parse e.g. `default use foo::bar;` and whatnot? Accepting that is likely easiest from a parsing perspective, as it does not require using look-ahead, but it is perhaps not too onerous to only accept it for `fn`s (and all their various qualifiers), `const`s, `static`s, and `type`s.

r? @petrochenkov