…tures (TS1221/TS1222) (#22848)

A generator (`function*` / `*method`) is not allowed in an ambient context
(TS1221), and a generator overload signature with no body is not allowed
(TS1222). Mirrors tsc's `checkGrammarForGenerator`:

    declare function* f(): any;              // now TS1221
    declare namespace N { function* g(); }   // now TS1221
    declare class C { *m(); }                // now TS1221
    function* f(): any;                       // overload → now TS1222
    function* f() { yield 1; }

    function* f() { yield 1; }               // normal generator → still valid

The check lives in `parse_function` next to the TS1183 implementation-in-ambient
check and keys off `ctx.has_ambient()` / `body.is_none()`. Class methods are
covered too (`class.rs` does not check generators, so there is no duplicate
diagnostic).

Also fixes a latent bug: `parse_ts_declare_function` hard-coded `generator: false`
and never consumed the `*`, so `declare function* f()` silently dropped the
generator flag. It now eats the `*` and threads the flag through (which is what
lets TS1221 fire there).

parser_typescript +7 negative-pass (generatorInAmbientContext1-4, generatorOverloads1-3);
positive/AST stay 100%.

### 🚀 Features

- 9c71f2e ast, codegen, formatter: Add `WithClauseKeyword::as_str`
helper and use it (#22791) (camc314)

### 🐛 Bug Fixes

- cf5769c parser: Reject TypeScript declarations in single-statement
context (#22827) (Boshen)
- c360fb6 parser: Reject generators in ambient contexts and overload
signatures (TS1221/TS1222) (#22848) (Boshen)
- cc60d8d parser: Reject invalid index signature parameter types
(TS1268/TS1337) (#22852) (Boshen)
- 3d13e29 parser: Reject `declare` in an already-ambient context
(TS1038) (#22850) (Boshen)
- 5152854 parser: Reject statements in ambient contexts (TS1036)
(#22849) (Boshen)
- 4f9afc5 parser: Reject `export as namespace` inside a namespace body
(TS1316) (#22846) (Boshen)
- 2eafea6 parser: Reject function implementations in ambient contexts
(TS1183) (#22845) (Boshen)
- c645615 parser: Reject incompatible class member modifiers (#22843)
(Boshen)
- 276b78b parser: Reject module-referencing imports/exports in a
namespace body (#22836) (Boshen)
- 842ed1c parser: Parse `class implements` with `implements` as the
class name (#22801) (Boshen)

### ⚡ Performance

- 7e3a567 parser: Reuse cached token kind in delimited-list loops
(#22841) (Boshen)
- 9e741c2 parser: Use peek_token instead of lookahead on the modifier
path (#22842) (Boshen)
- 9e496a7 semantic: Defer declare lookup for empty accessors (#22810)
(camc314)