Add: validation of bundled themes in build workflow (#11627)

* 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

xtask/src/theme_check.rs

@@ -0,0 +1,33 @@
+use helix_view::theme::Loader;
+
+use crate::{path, DynError};
+
+pub fn theme_check() -> Result<(), DynError> {
+    let theme_names = [
+        vec!["default".to_string(), "base16_default".to_string()],
+        Loader::read_names(&path::themes()),
+    ]
+    .concat();
+    let loader = Loader::new(&[path::runtime()]);
+    let mut errors_present = false;
+
+    for name in theme_names {
+        let (_, warnings) = loader.load_with_warnings(&name).unwrap();
+
+        if !warnings.is_empty() {
+            errors_present = true;
+            println!("Theme '{name}' loaded with errors:");
+            for warning in warnings {
+                println!("\t* {}", warning);
+            }
+        }
+    }
+
+    match errors_present {
+        true => Err("Errors found when loading bundled themes".into()),
+        false => {
+            println!("Theme check successful!");
+            Ok(())
+        }
+    }
+}