Add VERSION file as the single source for the extension version

Mirrors testium's src/VERSION: scripts/sync-version.js copies VERSION into
package.json#version (enforcing strict semver X.Y.Z, required by the marketplace
/ Open VSX). Wired into 'npm run package' and vscode:prepublish, so the version
is always synced before a build/publish. Bump = edit VERSION.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -51,7 +51,11 @@
         },
         "testium.trace.server": {
           "type": "string",
-          "enum": ["off", "messages", "verbose"],
+          "enum": [
+            "off",
+            "messages",
+            "verbose"
+          ],
           "default": "off",
           "description": "Trace the LSP communication between VSCode and the testium server in the Output panel."
         }
@@ -59,9 +63,11 @@
     }
   },
   "scripts": {
+    "sync-version": "node scripts/sync-version.js",
     "compile": "tsc -p .",
     "watch": "tsc -p . --watch",
-    "vscode:prepublish": "npm run compile"
+    "package": "npm run sync-version && npx --yes @vscode/vsce package",
+    "vscode:prepublish": "npm run sync-version && npm run compile"
   },
   "dependencies": {
     "vscode-languageclient": "^9.0.0"