feat(Schema) Add JSON schema for the extensions config files

[Cyclodex]

Closes #5

What

Ships a JSON schema for the config files this extension reads, so .vscode/extensionsVersionCheck.json(c) gets validation, autocompletion and hover documentation in the editor — the "built in help" the issue asked for, comparable to what VS Code offers for the original extensions.json.

It is contributed via contributes.jsonValidation, so it is pure metadata: no activation event, no runtime code, nothing to load.

"jsonValidation": [
  {
    "fileMatch": [
      "**/.vscode/extensions*.json",
      "**/.vscode/extensions*.jsonc",
      "!**/.vscode/extensions.json"
    ],
    "url": "./schemas/extensionsVersionCheck.schema.json"
  }
]

The fileMatch mirrors the discovery glob in src/utils.ts (**/.vscode/extensions*.{json,jsonc}), so every file the extension actually reads is covered.

Why .vscode/extensions.json is excluded

VS Code registers its own schema for that file and combines contributed schemas with the built-in one rather than replacing it. Its schema declares additionalProperties: false and restricts entries to plain publisher.name, so:

• a second schema on that file would only add duplicate diagnostics
• it would still flag publisher.name@1.0.0 entries as invalid, and we cannot suppress that

Excluding it keeps the diagnostics clean and matches what the README already recommends: version ranges belong in extensionsVersionCheck.json(c). Note that exclusion patterns must stay last in fileMatch — the last matching pattern wins.

What the schema gives you

• unknown properties and wrong types are reported
• entries are checked against ${publisher}.${name}[@versionRange], with a custom patternErrorMessage
• hover documentation on both arrays, including the supported range forms
• snippets for adding an entry with or without a version range

The version part is matched permissively on purpose. A strict SemVer regex would reject documented forms such as 0.1 - 0.9, 0.* or prerelease versions; real range validation is semver's job at runtime.

Verification

• Validated with vscode-json-languageservice (the very library VS Code uses): the schema resolves for extensionsVersionCheck.json(c) and other extensions* variants including subfolders, does not claim .vscode/extensions.json, and reports zero diagnostics on the exampleProject/ fixtures.
• vsce ls confirms schemas/extensionsVersionCheck.schema.json is included in the packaged VSIX.
• npm run compile, npm run lint and npm run test:unit all pass (32 tests, 6 new).

New unit tests in src/test/unit/schema.test.ts keep the contribution honest: the schema file exists and is referenced, the built-in extensions.json stays excluded, the fileMatch stays in sync with the discovery glob, and every entry used in the exampleProject/ fixtures is accepted by the pattern and is a valid SemVer range according to parseUnwantedEntry + semver.

Notes

• Based on develop, per the branching convention in CLAUDE.md.
• The CHANGELOG entry sits under [Unreleased] — no version bump, so src/config/releaseMessages.conf.ts is untouched. Happy to fold both into a release commit if you prefer.
• Possible follow-ups, out of scope here: a DiagnosticCollection that validates SemVer ranges (and would also reach extensions.json), completion of installed extension IDs, and publishing the schema to SchemaStore.

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