Fix(Language): Add language-configuration for comment toggle

[0Miles]

Summary

packages/vscode/package.json's contributes.languages entry for master-css had no configuration field — there was no language-configuration.json. As a result, when the cursor sits inside meta.embedded.block.master-css.class (the embedded scope created by master-css.injection-react.json / injection-js.json / etc. for className={classNames(...)} etc.), VS Code asks the master-css language for comment characters, gets nothing, and the Toggle Line Comment shortcut becomes a no-op. That is the exact "doesn't work at all" symptom in #361.

Changes

• packages/language/syntaxes/language-configuration.json (new) — lineComment: \"//\", blockComment: [\"/*\", \"*/\"], plus standard brackets, autoClosingPairs, surroundingPairs. The comment characters are JS-syntactic on purpose: when the cursor is inside an embedded master-css block within a TS/JS host file (the situation in the issue's screenshots), toggling produces a valid host-language comment.
• packages/language/src/declaration.ts — adds configuration: './syntaxes/language-configuration.json'. packages/vscode/generate.ts propagates declaration verbatim into the language entry, so this is the single source of truth.
• packages/vscode/package.json — pre-applies the same field so pnpm generate is a no-op after merge (and CI checks that compare hand-written vs generated stay green). Diff verified by re-running the generate logic locally — the rendered output matches byte-for-byte.
• packages/language/tests/language-configuration.test.mjs (new) — 4 dependency-free node --test cases: JSON parses, line+block comments are present, brackets/auto-closing pairs are non-empty, and declaration.ts references the file. Runs as node --test packages/language/tests/language-configuration.test.mjs. No vitest deps added so this doesn't conflict with the separate feat/language-grammar-tests branch that introduces vitest to this package.

Why this scope

The issue's second screenshot ("works but syntax highlighting broken") is a separate grammar concern — fixing the missing language-configuration is necessary regardless and is the cleanly-isolatable change. Keeping this PR surgical means it can land independently.

Test plan

• node --test packages/language/tests/language-configuration.test.mjs — 4/4 pass
• tsx -e re-render of generate.ts's language emission matches the hand-applied edit in packages/vscode/package.json
• Manual VS Code verification: install local VSIX, place cursor inside className={classNames(\"bg:white\", \"fg:black\")}, press Cmd/Ctrl+/, observe both lines toggle correctly

Closes #361

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