feat(lsp): configurable diagnostic severity (#1325)

* feat(lsp): configurable diagnostic severity

Allow severity of diagnostic messages to be configured.
E.g. allow turning of Hint level diagnostics.

Fixes: https://github.com/helix-editor/helix/issues/1007

* Use language_config() method

* Add documentation for diagnostic_severity

* Use unreachable for unknown severity level

* fix: documentation for diagnostic_severity config

helix-core/src/diagnostic.rs

@@ -1,12 +1,19 @@
 //! LSP diagnostic utility types.
+use serde::{Deserialize, Serialize};
 
 /// Describes the severity level of a [`Diagnostic`].
-#[derive(Debug, Clone, Copy, Eq, PartialEq)]
+#[derive(Debug, Clone, Copy, Eq, PartialEq, PartialOrd, Ord, Deserialize, Serialize)]
 pub enum Severity {
-    Error,
-    Warning,
-    Info,
     Hint,
+    Info,
+    Warning,
+    Error,
+}
+
+impl Default for Severity {
+    fn default() -> Self {
+        Self::Hint
+    }
 }
 
 /// A range of `char`s within the text.