scaffold: VSCode/VSCodium extension wrapping testium lsp

A thin LSP client that spawns `testium lsp` and forwards messages to
VSCode. All language intelligence lives in the testium repo, so this
extension only needs republishing for editor-side UX changes.

  - package.json: declares the .tum language, registers the LSP client,
    exposes testium.serverPath and testium.trace.server settings.
  - src/extension.ts: vscode-languageclient setup over stdio, surfaces a
    user-visible error if `testium lsp` can't start.
  - syntaxes/tum.tmLanguage.json: TextMate grammar embedding source.yaml
    plus tum-specific tokens for $(name), <| python |>, {% jinja %},
    {{ jinja }}.
  - language-configuration.json: comment char, bracket pairs, auto-close
    for $(/), <|/|>, {%/%}, {{/}}.
  - README.md: install (incl. testium[lsp] extra), settings, packaging
    via vsce, publish to both VS Marketplace and Open VSX (the latter
    is what VSCodium / Cursor / Gitpod use — same .vsix artifact).

Works identically on VSCode and VSCodium — no Microsoft-proprietary
API surface used.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tsconfig.json

@@ -0,0 +1,15 @@
+{
+  "compilerOptions": {
+    "module": "commonjs",
+    "target": "ES2022",
+    "lib": ["ES2022"],
+    "outDir": "out",
+    "rootDir": "src",
+    "sourceMap": true,
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", ".vscode-test"]
+}