parse: towards unified fn grammar

[Centril]

Part of #68728.

• Syntactically, fn items in extern { ... } blocks can now have bodies (fn foo() { ... } as opposed to fn foo();). As above, we use semantic restrictions instead.

• Syntactically, fn items in free contexts (directly in a file or a module) can now be without bodies (fn foo(); as opposed to fn foo() { ... }. As above, we use semantic restrictions instead, including for non-ident parameter patterns.

• We move towards unifying the fn front matter; this is fully realized in parse: merge fn syntax + cleanup item parsing #68728.

r? @petrochenkov