[webdev] Add support for --[no-]deprecated-js-interop - #2874

The Dart SDK added a `--[no-]deprecated-js-interop` flag to its web compilers (https://dart-review.googlesource.com/c/sdk/+/555460, https://dart-review.googlesource.com/c/sdk/+/555500). With `--no-deprecated-js-interop`, the compilers reject imports of `dart:html`, `dart:js`, `dart:js_util`, `package:js` and the other deprecated JS interop libraries.

This adds the flag to `webdev serve`, `webdev build` and `webdev daemon`. It matches the flag added to `flutter build web/run/drive/test`.

- **New flag:** `--[no-]deprecated-js-interop`, with `--help` text. If it isn't passed, nothing is forwarded and the SDK default applies, so older SDKs keep working.
- **Forwarded to build_runner** as `--define` for the four build_web_compilers builders that must agree on it: `ddc`, `ddc_modules`, `dart2js_modules` and `entrypoint`. This covers DDC, dart2js, the kernel worker and the Frontend Server (web hot reload). Changing the value invalidates cached outputs through build_runner's option digest.
- **Forwarded to dwds** through `BuildSettings`, so expression evaluation uses the same setting.
- **Version check.** If the flag is passed and the project's `build_web_compilers` is older than 4.9.0, webdev stops early with "could not run with --no-deprecated-js-interop for this project" instead of failing inside build_runner.
- **Error output.** Compiler diagnostics, including dart2js's import tree, come from build_web_compilers and are printed as `[SEVERE]` build logs. No change to webdev's logging.
- README help output is updated.

Depends on DWDS PR: dart-lang#2875