Use more LSP data when falling back to regular completions label (#23909)

Closes https://github.com/zed-industries/zed/issues/23590
Closes https://x.com/steeve/status/1865129235536568555

Before:

<img width="773" alt="before"
src="https://github.com/user-attachments/assets/129a8d12-9298-4bf5-8f2d-b3292c2562bf"
/>


After:

<img width="768" alt="after"
src="https://github.com/user-attachments/assets/e0516fb3-b02a-48be-8923-63bba05fdb69"
/>


The list obviously needs some solution for the cut-off part of the
completion label, but this is the reality for all extensions'
completions too, so one step at a time.


Release Notes:

- Improved default completion label fallback

crates/language/src/language.rs

@@ -1750,6 +1750,58 @@ impl Grammar {
 }
 
 impl CodeLabel {
+    pub fn fallback_for_completion(
+        item: &lsp::CompletionItem,
+        language: Option<&Language>,
+    ) -> Self {
+        let highlight_id = item.kind.and_then(|kind| {
+            let grammar = language?.grammar()?;
+            use lsp::CompletionItemKind as Kind;
+            match kind {
+                Kind::CLASS => grammar.highlight_id_for_name("type"),
+                Kind::CONSTANT => grammar.highlight_id_for_name("constant"),
+                Kind::CONSTRUCTOR => grammar.highlight_id_for_name("constructor"),
+                Kind::ENUM => grammar
+                    .highlight_id_for_name("enum")
+                    .or_else(|| grammar.highlight_id_for_name("type")),
+                Kind::FIELD => grammar.highlight_id_for_name("property"),
+                Kind::FUNCTION => grammar.highlight_id_for_name("function"),
+                Kind::INTERFACE => grammar.highlight_id_for_name("type"),
+                Kind::METHOD => grammar
+                    .highlight_id_for_name("function.method")
+                    .or_else(|| grammar.highlight_id_for_name("function")),
+                Kind::OPERATOR => grammar.highlight_id_for_name("operator"),
+                Kind::PROPERTY => grammar.highlight_id_for_name("property"),
+                Kind::STRUCT => grammar.highlight_id_for_name("type"),
+                Kind::VARIABLE => grammar.highlight_id_for_name("variable"),
+                Kind::KEYWORD => grammar.highlight_id_for_name("keyword"),
+                _ => None,
+            }
+        });
+
+        let label = &item.label;
+        let label_length = label.len();
+        let runs = highlight_id
+            .map(|highlight_id| vec![(0..label_length, highlight_id)])
+            .unwrap_or_default();
+        let text = if let Some(detail) = &item.detail {
+            format!("{label} {detail}")
+        } else if let Some(description) = item
+            .label_details
+            .as_ref()
+            .and_then(|label_details| label_details.description.as_ref())
+        {
+            format!("{label} {description}")
+        } else {
+            label.clone()
+        };
+        Self {
+            text,
+            runs,
+            filter_range: 0..label_length,
+        }
+    }
+
     pub fn plain(text: String, filter_text: Option<&str>) -> Self {
         let mut result = Self {
             runs: Vec::new(),