Refactor regex compilation and use is_some_and

This commit introduces several improvements:

- Pre-compiles regular expressions in `ast/tester.rs` to avoid redundant
  compilation, improving performance.
- Replaces `.extension().map_or(false, |ext| ext == "myc")` with the
  more concise and readable `.extension().is_some_and(|ext| ext ==
  "myc")`.
- Simplifies upvalue capture logic by using `.or_default()` instead of
  `.or_insert_with(HashSet::new)`.
- Adds a `Default` implementation for `TracingObserver`.
- Optimizes string literal parsing in `highlight_myc` to correctly
  handle escape sequences.
- Uses `saturating_sub` for bracket depth to prevent underflow.

src/ast/vm.rs

@@ -53,6 +53,12 @@ impl TracingObserver {
     }
 }
 
+impl Default for TracingObserver {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl VMObserver for TracingObserver {
     const ACTIVE: bool = true;