bundler: inline ungate_support into bundle_v2 and lib.rs, delete the file (#31529)

## What

`src/bundler/ungate_support.rs` was a porting-reference grab-bag: value
types extracted from `bundle_v2.zig` so the sibling `.rs` modules could
compile, plus a set of crate-name alias shims, all glob-re-exported at
the crate root via `pub use ungate_support::*` in `lib.rs`.

This dissolves that file into its natural homes and deletes it.

## How

**Value types → `bundle_v2.rs`** (next to their `bundle_v2.zig`
counterparts, inside the existing `bv2_impl` module that `bundle_v2.rs`
already re-exported them through):

`PartRange`, `StableRef`, `ImportTracker`, `CompileResult`,
`CompileResultForSourceMap`(+columns), `ContentHasher`, `WrapKind`,
`DeclInfo`/`DeclInfoKind`, `CrossChunkImport`/`CrossChunkImportItem`,
the `entry_point` and `js_meta` submodules, and the free functions
`generic_path_with_pretty_initialized` / `target_from_hashbang` /
`write_escaped_namespace`.

**Crate-name alias shims → `lib.rs` crate root** (where the other
cross-cutting re-exports like `Index`, `options`, `bake_types` already
live):

`bun_fs`, `bun_css`, `perf`, `bun_renamer`, `html_import_manifest`,
`html_scanner`, `cross_chunk_import`, `index`, `part`, `import_record`,
`JSAst`, `UseDirective`, `MangledProps`, `PathTemplate`, `EventLoop`.

**Dropped** the unused `bun_str` / `bun_output` aliases (zero
references).

All porting-reference comments in the moved code are removed, and the
now-stale comments elsewhere that referenced `ungate_support` are
cleaned up.

## No behavior change

This is a pure relocation. Every item keeps its original visibility
(`pub`/`pub(crate)`) and the crate-root spellings callers use
(`crate::bun_fs`, `crate::WrapKind`, `crate::js_meta::Flags`, …) are
unchanged — the ~50 call sites were only repointed from
`crate::ungate_support::X` to `crate::X` where the glob previously
placed them.

## Verification

- `cargo check -p bun_bundler`: clean, zero warnings
- `bun bd`: full debug build succeeds
- `bun bd test test/bundler/bundler_edgecase.test.ts`: 103 pass / 0 fail
- `bun bd test test/bundler/bundler_splitting.test.ts` (cross-chunk CSS,
entry points — exercises the moved
`CrossChunkImport`/`StableRef`/`ContentHasher`): 10 pass / 0 fail