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/editor/src/movement.rs

@@ -772,7 +772,7 @@ mod tests {
     };
     use gpui::{AppContext as _, font, px};
     use language::Capability;
-    use project::Project;
+    use project::{Project, project_settings::DiagnosticSeverity};
     use settings::SettingsStore;
     use util::post_inc;
 
@@ -896,6 +896,7 @@ mod tests {
                 1,
                 1,
                 FoldPlaceholder::test(),
+                DiagnosticSeverity::Warning,
                 cx,
             )
         });
@@ -1105,6 +1106,7 @@ mod tests {
                     0,
                     1,
                     FoldPlaceholder::test(),
+                    DiagnosticSeverity::Warning,
                     cx,
                 )
             });