fix(example): mount monaco directly instead of react-monaco-editor, cover example in CI

[murrayju]

Problem

yarn build in ./example fails, which breaks the GitHub Pages deploy at the end of the publish workflow:

Module not found: Error: Can't resolve 'monaco-editor/esm/vs/editor/editor.api'
  in '.../node_modules/react-monaco-editor/lib'

react-monaco-editor does import * as monaco from "monaco-editor/esm/vs/editor/editor.api", but monaco-editor 0.56 added an exports map:

"./*": "./esm/vs/*.js"

That rewrites the old specifier to ./esm/vs/esm/vs/editor/editor.api.js, which the package doesn't ship. This is upstream fallout from the monaco 0.56 upgrade in 1d50b6a — react-monaco-editor's latest release (0.59.0) still peer-depends on monaco-editor@^0.52.0 and hasn't adapted, so there's no version to upgrade to.

Changes

Drop react-monaco-editor and mount the editor directly with monaco.editor.create, importing via the supported monaco-editor/editor/editor.api specifier.

• Language switching now uses monaco.editor.setModelLanguage on the existing model, so the editor's contents survive a dialect change (previously the wrapper remounted with reset state).
• Language registration is guarded to run once, since it mutates global monaco state.
• MonacoEnvironment.getWorker is set in index.js; without it monaco tries to load its editor worker from a CDN-style absolute path that doesn't exist here. webpack 5 bundles the worker from the new URL(..., import.meta.url) form.
• The language dropdown is populated from the definition list at module scope rather than being filled in as a side effect of the mount callback.
• Removed example/src/App.css — unreferenced CRA boilerplate (spinning-logo styles).

Add an example job to CI. The example consumes the package through file:../, so it's the only check that exercises the built entry points and the monaco-editor peer dependency the way a real consumer does. The publish workflow builds it to deploy Pages, so without this job a break is only discovered mid-release — which is exactly what happened here. react-scripts runs eslint as part of a production build, so the job covers lint and compilation.

Verification

• yarn lint, yarn test (228 passed), and yarn build pass at the root; yarn install --frozen-lockfile + yarn build pass in ./example, all from a clean clone of this branch.
• Served the production build and loaded it in Chrome: editor renders, no console errors.
• Confirmed tokenization works for all six dialects by reading the rendered token classes after switching each one — including pgsql, which lazy-loads from monaco's own language definitions.
• Confirmed editing works by typing WHERE created_at > now() via real keyboard events and checking the resulting tokens.
• Confirmed the web worker actually loads and executes at runtime (not merely that an asset was emitted) by invoking MonacoEnvironment.getWorker and watching for error/messageerror plus the extra chunk fetch.
• yarn start compiles successfully.
• Sanity-checked that the new CI job catches this regression class: reverting just the import specifier back to the old path makes the example job fail with the original error.

Note

The 117 Dependabot alerts on this repo are pre-existing (mostly react-scripts 5.0.1's transitive dev deps) and out of scope here.