Rollup merge of #149174 - GrigorenkoPV:const_block_item, r=me,ytmimi

`const` blocks as a `mod` item

Tracking issue: rust-lang/rust#149226

This adds support for writing `const { ... }` as an item in a module. In the current implementation, this is a unique AST item that gets lowered to `const _: () = const { ... };` in HIR.

rustfmt support included.

TODO:
- `pub const { ... }` does not make sense (see rust-lang/rust#147136). Reject it. Should this be rejected by the parser or smth?
- Improve diagnostics (preferably they should not mention the fake `_` ident).

Author: matthiaskrgr