### What does this PR do?
Previously, reactFastRefresh was silently ignored when target was not
'browser', even when explicitly enabled. This was confusing as there was
no warning or error.
This change removes the `target == .browser` check, trusting explicit
user intent. If users enable reactFastRefresh with a non-browser target,
the transform will now be applied. If `$RefreshReg$` is not defined at
runtime, it will fail fast with a clear error rather than silently doing
nothing.
Use case: Terminal UIs (like [termcast](https://termcast.app)) need
React Fast Refresh with target: 'bun' for hot reloading in non-browser
environments.
### How did you verify your code works?
Updated existing test removing target browser
Fixes#25716
Adds support for a `reactFastRefresh: boolean` option in the `Bun.build`
JavaScript API, matching the existing `--react-fast-refresh` CLI flag.
```ts
const result = await Bun.build({
reactFastRefresh: true,
entrypoints: ["src/App.tsx"],
});
```
When enabled, the bundler adds React Fast Refresh transform code
(`$RefreshReg$`, `$RefreshSig$`) to the output.
