* Add: xtask to check themes for validation warnings
* Update: tidied up runtime paths
* Update: test build workflow
* Update: address clippy lints
* Revert: only trigger workflow on push to master branch
* Add: Theme::from_keys factory method to construct theme from Toml keys
* Update: returning validation failures in Loader.load method
* Update: commented out invalid keys from affected themes
* Update: correct invalid keys so that valid styles still applied
* Update: include default and base16_default themes in check
* Update: renamed validation_failures to load_errors
* Update: introduce load_with_warnings helper function and centralise logging of theme warnings
* Update: use consistent naming throughout
