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/test.rs

@@ -18,7 +18,7 @@ use gpui::{
 };
 use multi_buffer::ToPoint;
 use pretty_assertions::assert_eq;
-use project::Project;
+use project::{Project, project_settings::DiagnosticSeverity};
 use ui::{App, BorrowAppContext, px};
 use util::test::{marked_text_offsets, marked_text_ranges};
 
@@ -72,6 +72,7 @@ pub fn marked_display_snapshot(
             1,
             1,
             FoldPlaceholder::test(),
+            DiagnosticSeverity::Warning,
             cx,
         )
     });