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/main.rs

@@ -46,7 +46,7 @@ impl Default for CompilerApp {
             .map(|entries| {
                 entries
                     .filter_map(|e| e.ok())
-                    .filter(|e| e.path().extension().map_or(false, |ext| ext == "myc"))
+                    .filter(|e| e.path().extension().is_some_and(|ext| ext == "myc"))
                     .filter_map(|e| {
                         let path = e.path();
                         let name = e.file_name().into_string().ok()?;
@@ -184,7 +184,7 @@ impl CompilerApp {
         if let Ok(entries) = std::fs::read_dir("examples") {
             self.examples = entries
                 .filter_map(|e| e.ok())
-                .filter(|e| e.path().extension().map_or(false, |ext| ext == "myc"))
+                .filter(|e| e.path().extension().is_some_and(|ext| ext == "myc"))
                 .filter_map(|e| {
                     let path = e.path();
                     let name = e.file_name().into_string().ok()?;
@@ -445,8 +445,8 @@ fn highlight_myc(ctx: &egui::Context, code: &str) -> egui::text::LayoutJob {
                 while let Some(next) = chars.next() {
                     text.push(next);
                     if next == '"' { break; }
-                    if next == '\\' {
-                        if let Some(escaped) = chars.next() { text.push(escaped); }
+                    if next == '\\' && let Some(escaped) = chars.next() {
+                        text.push(escaped);
                     }
                 }
                 color = color_str;
@@ -463,7 +463,7 @@ fn highlight_myc(ctx: &egui::Context, code: &str) -> egui::text::LayoutJob {
                 bracket_depth += 1;
             }
             ')' | ']' | '}' => {
-                if bracket_depth > 0 { bracket_depth -= 1; }
+                bracket_depth = bracket_depth.saturating_sub(1);
                 color = color_bracket[bracket_depth % 3];
             }
             '0'..='9' => {