Highlight bash conditionals and arithmetic operators (#56484)

Resolves https://github.com/zed-industries/zed/issues/56478

This diff fixes Bash syntax highlighting for conditional and arithmetic
expressions. The Bash parser already recognizes tokens like `[[`, `]]`,
`((`, `))`, `$((`, `!=`, and `||`, but Zed’s Bash highlight query was
not capturing several of them.

With this change, the Bash highlight query styles those operators and
delimiters consistently with existing shell operators, brackets, and
special punctuation.

| Before | After |
| --- | --- |
| <img width="1667" height="1088" alt="ayu-old"
src="https://github.com/user-attachments/assets/0a89bd71-0a13-4ebe-8bc9-abc27ab42ed8"
/> | <img width="1667" height="1088" alt="ayu-new"
src="https://github.com/user-attachments/assets/afa5e8ab-f94a-4f5e-8fe8-770bac07559a"
/> |

Release Notes:
- Fixed bash syntax highlighting for conditional expressions, arithmetic
expressions, and related operators.

crates/grammars/src/bash/highlights.scm

@@ -72,6 +72,7 @@
 [
   "$"
   "&&"
+  "||"
   ">"
   "<<"
   ">>"
@@ -86,8 +87,36 @@
   "%%"
   "#"
   "##"
+  "+="
+  "-="
+  "*="
+  "/="
+  "%="
+  "**="
+  "<<="
+  ">>="
+  "&="
+  "^="
+  "|="
   "="
+  "=~"
   "=="
+  "!="
+  "-o"
+  "-a"
+  "^"
+  "&"
+  "<="
+  ">="
+  "+"
+  "-"
+  "*"
+  "**"
+  "!"
+  "++"
+  "--"
+  "~"
+  "?"
 ] @operator
 
 (test_operator) @keyword.operator
@@ -103,6 +132,18 @@
   "]"
 ] @punctuation.bracket
 
+(test_command
+  [
+    "[["
+    "]]"
+  ] @punctuation.bracket)
+
+(compound_statement
+  [
+    "(("
+    "))"
+  ] @punctuation.bracket)
+
 (simple_expansion
   "$" @punctuation.special)
 
@@ -114,6 +155,14 @@
   "$(" @punctuation.special
   ")" @punctuation.special)
 
+(arithmetic_expansion
+  [
+    "$(("
+    "$["
+    "))"
+    "]"
+  ] @punctuation.special)
+
 ((command
   (_) @constant)
   (#match? @constant "^-"))