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>

.vscodeignore

@@ -0,0 +1,12 @@
+.vscode/**
+.vscode-test/**
+src/**
+.gitignore
+.git
+tsconfig.json
+node_modules/.cache/**
+**/*.map
+**/*.ts
+!out/**/*.js
+.github/**
+.editorconfig