Improve LSP tasks ergonomics (#31551)

* stopped fetching LSP tasks for too long (but still use the hardcoded
value for the time being — the LSP tasks settings part is a simple bool
key and it's not very simple to fit in another value there)

* introduced `prefer_lsp` language task settings value, to control
whether in the gutter/modal/both/none LSP tasks are shown exclusively,
if possible

Release Notes:

- Added a way to prefer LSP tasks over Zed tasks

crates/markdown/src/markdown.rs

@@ -225,7 +225,7 @@ impl Markdown {
         self.parse(cx);
     }
 
-    #[cfg(feature = "test-support")]
+    #[cfg(any(test, feature = "test-support"))]
     pub fn parsed_markdown(&self) -> &ParsedMarkdown {
         &self.parsed_markdown
     }