Two post-stream follow-ups flagged in CHECKPOINT-EXTERN-EXPORT.md, plus a
reproducible vscode-extension packaging setup:
- parser: drop the vestigial `RuntimeClassPrefix.is_extern` field and
`parseRuntimeClassDecl`'s `is_extern` param. Always false since the
`#foreign` token was deleted; the postfix `extern`/`export` keyword is the
sole reference-vs-define decider. No behavior change (644 corpus / 442 unit).
- vscode grammar: highlight `extern`/`export` as `storage.modifier.sx`.
- vscode packaging: declare `@vscode/vsce` as a devDep + add `package` /
`vscode:prepublish` scripts so the vsix rebuilds reproducibly (was an
ambient tool). Add repository/homepage/bugs (Gitea), icon (swipelab logo,
256x256), galleryBanner, README with cover banner. Rebuilt the vsix.
VSCode disables the breakpoint gutter for a language unless an extension
declares breakpoints are valid for it. The sx extension registered the
language but never contributed breakpoints, so clicking the gutter in a
.sx file did nothing. Add the breakpoints contribution so users can set
breakpoints in .sx files without the per-workspace
debug.allowBreakpointsEverywhere hack.
