Implement RFC 3503: frontmatters

[fee1-dead]

Supercedes #137193. This implements RFC 3503.

This might break rust-analyzer. Will look into how to fix that.

Suggestions welcome for how to improve diagnostics.

[rustbot]

r? @jieyouxu

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] stdx test:false 0.210
    Checking parser v0.0.0 (/checkout/src/tools/rust-analyzer/crates/parser)
[RUSTC-TIMING] chalk_derive test:false 0.840
    Checking tt v0.0.0 (/checkout/src/tools/rust-analyzer/crates/tt)
error[E0004]: non-exhaustive patterns: `&TokenKind::Frontmatter { .. }` not covered
   --> src/tools/rust-analyzer/crates/parser/src/lexed_str.rs:171:19
    |
171 |             match kind {
    |                   ^^^^ pattern `&TokenKind::Frontmatter { .. }` not covered
    |
note: `TokenKind` defined here
   --> /checkout/compiler/rustc_lexer/src/lib.rs:58:1
    |
58  | pub enum TokenKind {
    | ^^^^^^^^^^^^^^^^^^
...
76  |     Frontmatter {
    |     ----------- not covered
    = note: the matched value is of type `&TokenKind`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
254 ~                 rustc_lexer::TokenKind::Eof => EOF,
255 ~                 &TokenKind::Frontmatter { .. } => todo!(),
    |

For more information about this error, try `rustc --explain E0004`.
[RUSTC-TIMING] parser test:false 0.378
error: could not compile `parser` (lib) due to 1 previous error

[jieyouxu]

I'll do a review pass next Monday/Tuesday, but in the meantime, I'll roll this to Wesley who I think have more context on the general vibes.

r? @wesleywiser