Allow to temporarily toggle diagnostics in the editor (#30316)

* Adds a `diagnostics_max_severity: null` editor settings that has
previous hardcoded default, `warning`
* Make inline diagnostics to inherit this setting by default (can be
overridden with its own max_severity setting)
* Allows to toggle diagnostics in the editor menu and via new action,
`editor::ToggleDiagnostics`

Closes https://github.com/zed-industries/zed/issues/4686

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/project/src/project_settings.rs

@@ -181,15 +181,31 @@ pub struct CargoDiagnosticsSettings {
     pub fetch_cargo_diagnostics: bool,
 }
 
-#[derive(Clone, Copy, Debug, Serialize, Deserialize, JsonSchema)]
+#[derive(
+    Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd, Serialize, Deserialize, JsonSchema,
+)]
 #[serde(rename_all = "snake_case")]
 pub enum DiagnosticSeverity {
+    // No diagnostics are shown.
+    Off,
     Error,
     Warning,
     Info,
     Hint,
 }
 
+impl DiagnosticSeverity {
+    pub fn into_lsp(self) -> Option<lsp::DiagnosticSeverity> {
+        match self {
+            DiagnosticSeverity::Off => None,
+            DiagnosticSeverity::Error => Some(lsp::DiagnosticSeverity::ERROR),
+            DiagnosticSeverity::Warning => Some(lsp::DiagnosticSeverity::WARNING),
+            DiagnosticSeverity::Info => Some(lsp::DiagnosticSeverity::INFORMATION),
+            DiagnosticSeverity::Hint => Some(lsp::DiagnosticSeverity::HINT),
+        }
+    }
+}
+
 impl Default for InlineDiagnosticsSettings {
     fn default() -> Self {
         Self {