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 ...

assets/settings/default.json

@@ -218,6 +218,23 @@
   // 1. Do nothing: `none`
   // 2. Find references for the same symbol: `find_all_references` (default)
   "go_to_definition_fallback": "find_all_references",
+  // Which level to use to filter out diagnostics displayed in the editor.
+  //
+  // Affects the editor rendering only, and does not interrupt
+  // the functionality of diagnostics fetching and project diagnostics editor.
+  // Which files containing diagnostic errors/warnings to mark in the tabs.
+  // Diagnostics are only shown when file icons are also active.
+  // This setting only works when can take the following three values:
+  //
+  // Which diagnostic indicators to show in the scrollbar, their level should be more or equal to the specified severity level.
+  // Possible values:
+  //  - "off" — no diagnostics are allowed
+  //  - "error"
+  //  - "warning" (default)
+  //  - "info"
+  //  - "hint"
+  //  - null — allow all diagnostics
+  "diagnostics_max_severity": "warning",
   // Whether to show wrap guides (vertical rulers) in the editor.
   // Setting this to true will show a guide at the 'preferred_line_length' value
   // if 'soft_wrap' is set to 'preferred_line_length', and will show any
@@ -1002,7 +1019,7 @@
       // longer than this value will still push diagnostics further to the right.
       "min_column": 0,
       // The minimum severity of the diagnostics to show inline.
-      // Shows all diagnostics when not specified.
+      // Inherits editor's diagnostics' max severity settings when `null`.
       "max_severity": null
     },
     "cargo": {