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>

package.json

@@ -0,0 +1,69 @@
+{
+  "name": "testium-assist",
+  "displayName": "Testium Assist",
+  "description": "Language support for testium .tum test scripts. Completion, hover and diagnostics powered by the testium LSP server (`testium lsp`).",
+  "version": "0.1.0",
+  "publisher": "testium",
+  "license": "EUPL-1.2",
+  "engines": {
+    "vscode": "^1.80.0"
+  },
+  "categories": [
+    "Programming Languages"
+  ],
+  "main": "./out/extension.js",
+  "activationEvents": [
+    "onLanguage:tum"
+  ],
+  "contributes": {
+    "languages": [
+      {
+        "id": "tum",
+        "aliases": [
+          "Testium TUM",
+          "tum"
+        ],
+        "extensions": [
+          ".tum"
+        ],
+        "configuration": "./language-configuration.json"
+      }
+    ],
+    "grammars": [
+      {
+        "language": "tum",
+        "scopeName": "source.tum",
+        "path": "./syntaxes/tum.tmLanguage.json"
+      }
+    ],
+    "configuration": {
+      "title": "Testium Assist",
+      "properties": {
+        "testium.serverPath": {
+          "type": "string",
+          "default": "testium",
+          "description": "Path to the testium executable used to start the language server (`<path> lsp`). Defaults to looking up `testium` on $PATH."
+        },
+        "testium.trace.server": {
+          "type": "string",
+          "enum": ["off", "messages", "verbose"],
+          "default": "off",
+          "description": "Trace the LSP communication between VSCode and the testium server in the Output panel."
+        }
+      }
+    }
+  },
+  "scripts": {
+    "compile": "tsc -p .",
+    "watch": "tsc -p . --watch",
+    "vscode:prepublish": "npm run compile"
+  },
+  "dependencies": {
+    "vscode-languageclient": "^9.0.0"
+  },
+  "devDependencies": {
+    "@types/node": "^20",
+    "@types/vscode": "^1.80.0",
+    "typescript": "^5.4.0"
+  }
+}