…and#34489)

## Summary

`deno bundle` (and any other transpile consumer) currently emits a
spurious
module-level hoist for every class that uses a computed member key, even
when
the class has no decorators:

```ts
// main.ts
(() => {
  class A {
    *[Symbol.iterator]() {/* empty */}
  }
})();
```

```js
// before
(() => {
  var _computedKey;
  _computedKey = Symbol.iterator;
  class A {
    *[_computedKey]() {}
  }
})();
```

That `_computedKey = Symbol.iterator` is a top-level expression
statement, so
esbuild can't prove the IIFE is pure and refuses to tree-shake it.
(esbuild
already has well-known-symbol handling for the un-hoisted form.)

Root cause: swc's Ecma decorator pass
(`swc_ecma_transforms_proposal::decorator_2022_03`) walks every class
and
unconditionally calls `process_decorators_of_class_members`, which
rewrites any
non-trivial computed prop name into a hoisted `_computedKey` even when
neither
the class nor its members carry any decorators.

Fix: pre-scan the parsed program for any `Decorator` node before calling
`parsed_source.transpile(...)` in `deno_resolver::emit::transpile`. When
the
module has none, force the transpile options' `decorators` field to
`DecoratorsTranspileOption::None` so swc's decorator pass is skipped
entirely.
Modules that actually use decorators still take the full pass and behave
identically to before.

Verified locally:

```text
$ deno bundle --output=out.js main.ts   # the issue's reproducer
Bundled 1 module in 29ms
  out.js 0B
```

The second reproducer in the issue (a plain `[""] = ""` computed field)
also
tree-shakes to `0B`, and a class with an actual `@dec` method still
bundles
and runs correctly (decorator runtime is preserved).

Closes denoland#30817.
Closes denoland/orchid#253

## Test plan

- [ ] `tests/specs/bundle/tree_shake_computed_key` exercises the
original
      reproducer and asserts that the bundled output contains neither
`_computedKey` nor `class A` (the IIFE has no other side effects, so
      esbuild can drop the whole thing).
- [ ] Existing bundle/decorator specs unaffected.

---------

Co-authored-by: divybot <[email protected]>
Co-authored-by: Divy Srivastava <[email protected]>