Add shared Stack language assets package

[konojunya]

背景

Stack source now has multiple editor-facing consumers: the Web playground, future documentation, and future editor integrations. They need one versioned lexical grammar without moving language ownership into a product or future LSP repository.

実装方法

• add ADR-0004 to keep shared editor language assets in the specification repository
• add the dependency-free @stack-sh/language package with a TextMate grammar, language configuration, JavaScript entry point, and TypeScript declarations
• cover every Stack 1.0 contextual keyword and verify tokenization with Shiki's pure JavaScript regex engine
• add package-content auditing, CI gates, and a language-v* release workflow for future npm Trusted Publishing

テスト方法

• npm ci
• npm run test:language
• npm run typecheck:language
• npm run pack:check
• all existing JSON Schema and fixture validation commands
• oxfmt --check and oxlint for package sources

Review 視点 / 懸念点

• The normative EBNF remains authoritative; TextMate scopes are presentation-only.
• Contextual keyword-shaped identifiers are intentionally highlighted lexically as keywords.
• The first public npm publish is not performed by this PR. It remains a separate explicit release step.