docs: document @keyword.control.exception scope

As identified in [this GitHub comment](https://github.com/helix-editor/helix/pull/1433#discussion_r777786140)

book/src/themes.md

@@ -147,6 +147,7 @@ #### Syntax highlighting
     - `repeat` - `for`, `while`, `loop`
     - `import` - `import`, `export`
     - `return`
+    - `exception`
   - `operator` - `or`, `in`
   - `directive` - Preprocessor directives (`#if` in C) 
   - `function` - `fn`, `func`