feat: Upgrade @rspack/core to v2 and fix breaking changes

[yoannmoinet]

What and why?

Fixes #418.

• Widens the @datadog/rspack-plugin peer dependency from "1.x" to "1.x || 2.x" so users on rspack v2 are no longer blocked.
• Upgrades the internal @rspack/core dev/test dependency from 1.4.9 → 2.0.8.
• Fixes the two rspack v2 breaking changes that affected this codebase.
• Fixes a Jest ESM incompatibility introduced by rspack v2 being pure ESM.

How?

Rspack v2 breaking changes addressed:

• experiments.css removed — rspack v2 dropped this option (CSS support is now always on). Removed it from configXpack() in packages/tools/src/bundlers.ts; leaving it in caused rspack to throw on unknown config.
• Explicit CSS module rule required — rspack v2 requires { test: /\.css$/, type: 'css' } in module.rules for CSS imports to be processed. Added it alongside the existing ts-loader rule so the easy_project_with_css fixture tests continue to work.

Jest ESM compatibility:

@rspack/core v2 ships as a pure ESM package ("type": "module"). The test suite runs Jest with --experimental-vm-modules, which causes Jest's shouldLoadAsEsm() to return true for any package with "type": "module" — making require('@rspack/core') throw ERR_REQUIRE_ESM before the transformer ever runs.

Fix: added a custom Jest resolver (rspack-jest-resolver.cjs) that intercepts require('@rspack/core') and redirects it to a .cjs shim (rspack-cjs-shim.cjs). Two properties make this work:

1. The .cjs extension causes shouldLoadAsEsm() to return false unconditionally, bypassing Jest's ESM guard.
2. Inside the shim, createRequire() from node:module is used instead of the module's own require. createRequire() is not patched by Jest and uses Node's native loader, which supports require(esm) natively on Node ≥ 20.17.

No test logic was changed; all pre-existing failures (macOS fsmonitor--daemon.ipc socket-copy errors in git integration tests) are unrelated to this PR and reproduce identically with the webpack bundler.