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/display_map/fold_map.rs

@@ -4,7 +4,6 @@ use super::{
 };
 use gpui::{AnyElement, App, ElementId, HighlightStyle, Pixels, Window};
 use language::{Edit, HighlightId, Point, TextSummary};
-use lsp::DiagnosticSeverity;
 use multi_buffer::{
     Anchor, AnchorRangeExt, MultiBufferRow, MultiBufferSnapshot, RowInfo, ToOffset,
 };
@@ -1253,7 +1252,7 @@ pub struct Chunk<'a> {
     /// the editor.
     pub highlight_style: Option<HighlightStyle>,
     /// The severity of diagnostic associated with this chunk, if any.
-    pub diagnostic_severity: Option<DiagnosticSeverity>,
+    pub diagnostic_severity: Option<lsp::DiagnosticSeverity>,
     /// Whether this chunk of text is marked as unnecessary.
     pub is_unnecessary: bool,
     /// Whether this chunk of text was originally a tab character.