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>

language-configuration.json

@@ -0,0 +1,38 @@
+{
+  "comments": {
+    "lineComment": "#"
+  },
+  "brackets": [
+    ["{", "}"],
+    ["[", "]"],
+    ["(", ")"]
+  ],
+  "autoClosingPairs": [
+    { "open": "{", "close": "}" },
+    { "open": "[", "close": "]" },
+    { "open": "(", "close": ")" },
+    { "open": "\"", "close": "\"", "notIn": ["string"] },
+    { "open": "'", "close": "'", "notIn": ["string"] },
+    { "open": "$(", "close": ")" },
+    { "open": "<|", "close": "|>" },
+    { "open": "{%", "close": "%}" },
+    { "open": "{{", "close": "}}" }
+  ],
+  "surroundingPairs": [
+    ["{", "}"],
+    ["[", "]"],
+    ["(", ")"],
+    ["\"", "\""],
+    ["'", "'"]
+  ],
+  "indentationRules": {
+    "increaseIndentPattern": "^.*:\\s*$",
+    "decreaseIndentPattern": "^\\s+[}\\]].*$"
+  },
+  "onEnterRules": [
+    {
+      "beforeText": "^\\s*-\\s+[^:]*:\\s*$",
+      "action": { "indent": "indent" }
+    }
+  ]
+}