Provide code fix data in JSON output

[colinkiama]

Now that we have a JSON output available via the j flag, we can now add fix data to allow code editor linter extensions like vala-lint-vscode and Sublime Linter Vala Lint to provide options in code editors to fix mistakes.

For examples of how linter extensions can work with JSON outputs to add actions in code editors, please see: https://github.com/fnando/vscode-linter/blob/main/docs/creating-linters.md#adding-code-actions