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/actions.rs

@@ -425,6 +425,7 @@ actions!(
         ToggleAutoSignatureHelp,
         ToggleGitBlameInline,
         OpenGitBlameCommit,
+        ToggleDiagnostics,
         ToggleIndentGuides,
         ToggleInlayHints,
         ToggleInlineValues,