fix(themes): declare generated runtime dependencies

[dahukish]

Problem

Theme packages whose generated ESM/CJS output imports @babel/runtime helpers or @lezer/highlight directly were missing those declarations in package.json. Under bundler configs that don't hoist phantom dependencies — raw webpack, pnpm strict mode, Vite library mode — consumers get a hard module-not-found error at build time.

Closes #755

Summary

• Declare @babel/runtime: ^7.18.6 in every theme package whose built output emits Babel helper require() calls.
• Declare @lezer/highlight: ^1.0.0 in every theme package whose source imports tags directly (all except console, which re-exports only from @uiw/codemirror-themes).
• Add themes/_scripts/check-runtime-dependencies.cjs — scans built esm/ and cjs/ output for imports not declared in package.json and exits non-zero on any gap.
• Wire the check into .github/workflows/pr.yml after npm run build to prevent regressions.

Note on @babel/runtime

The correct long-term fix is to configure the babel build to use inline helpers instead of @babel/runtime, removing the runtime dependency entirely. That change is out of scope here; this PR declares the existing implicit dependency explicitly.

Test plan

• npm run build followed by npm run check:theme-dependencies passes
• Negative test: removing @babel/runtime from themes/basic/package.json causes the check to fail as expected

[dahukish]

Hey @jaywcjlove 👋 — this fixes the Module not found: @babel/runtime crash reported in #755. Theme packages were missing runtime dependency declarations so bundlers without phantom-dep hoisting (raw webpack, pnpm strict mode) would blow up on install.

Two commits: declare the missing deps across all theme packages, and add a CI check to pr.yml that scans built output so it can't regress.

Happy to adjust anything — thanks for taking a look!