Feature/ shared cppcheck.json settings file

[danmar]

Adds support for a cppcheck.json file in the workspace root that customers can commit to their repository, so that all developers get the same cppcheck arguments without having to share their whole .vscode/settings.json.

{
  "arguments": "--project=build/compile_commands.json --enable=style"
}

• arguments may be a string or an array of strings.
• If the file specifies arguments, it takes precedence over the cppcheck-official.arguments setting. Other settings (enable, path, minSeverity) are still read from the VS Code settings, so each developer can tweak them individually.
• The file is read on each analysis, so changes (e.g. after a git pull) apply without reloading.
• If the file is invalid, an error is shown once (not again until the error changes) and the VS Code setting is used.
• A JSON schema is contributed (schemas/cppcheck.schema.json) for autocomplete and validation when editing cppcheck.json.
• README and walkthrough updated.

Tested:

• npm run compile and npm run lint pass
• Unit tests in src/test/settingsFile.test.ts (run in CI via npm test): no file, string, array, empty arguments, missing arguments, file changes, invalid JSON, wrong type, non-object file, and error reporting after the file is fixed and broken again

Not yet tested in the Extension Development Host.

🤖 Generated with Claude Code