chore(ffi-linkage): post-stream polish — vscode keywords + vestigial param + extension metadata

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.

editors/vscode/package.json

@@ -4,6 +4,19 @@
   "description": "Language support for the sx programming language",
   "version": "0.0.1",
   "publisher": "swipelab",
+  "icon": "icon.png",
+  "galleryBanner": {
+    "color": "#000000",
+    "theme": "dark"
+  },
+  "repository": {
+    "type": "git",
+    "url": "https://git.swipelab.com/lab/sx.git"
+  },
+  "homepage": "https://git.swipelab.com/lab/sx",
+  "bugs": {
+    "url": "https://git.swipelab.com/lab/sx/issues"
+  },
   "engines": {
     "vscode": "^1.75.0"
   },
@@ -73,13 +86,16 @@
   },
   "scripts": {
     "build": "tsc -p .",
-    "watch": "tsc -watch -p ."
+    "watch": "tsc -watch -p .",
+    "vscode:prepublish": "npm run build",
+    "package": "vsce package --baseContentUrl https://git.swipelab.com/lab/sx/src/branch/master/editors/vscode --baseImagesUrl https://git.swipelab.com/lab/sx/raw/branch/master/editors/vscode"
   },
   "dependencies": {
     "vscode-languageclient": "^9.0.1"
   },
   "devDependencies": {
     "@types/vscode": "^1.75.0",
+    "@vscode/vsce": "^3.9.2",
     "typescript": "^5.0.0"
   }
 }