feat: improve tracing

`--trace` now outputs a JSON structure with tracing information, eg.

```
{"lexer":"markdown","state":"root","rule":15,"pos":0,"elapsedMs":0.022875}
{"lexer":"markdown","state":"root","rule":15,"pos":1,"elapsedMs":0.002667}
{"lexer":"markdown","state":"root","rule":15,"pos":2,"elapsedMs":0.001833}
{"lexer":"markdown","state":"root","rule":15,"pos":3,"elapsedMs":0.002166}
{"lexer":"markdown","state":"root","rule":15,"pos":4,"elapsedMs":0.002125}
```

This should generally be much more amenable to analysis, eg. convenient
filtering using jq to help track down hotspots:

```
chroma --trace docs.md 2>&1 > /dev/null | jq 'select(. | .elapsedMs > 0.1)' | less
```

regexp.go

@@ -1,6 +1,7 @@
 package chroma
 
 import (
+	"encoding/json"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -135,11 +136,20 @@ func NewLexer(config *Config, rulesFunc func() Rules) (*RegexLexer, error) {
 }
 
 // Trace enables debug tracing.
+//
+// Deprecated: Use SetTracing instead.
 func (r *RegexLexer) Trace(trace bool) *RegexLexer {
 	r.trace = trace
 	return r
 }
 
+// SetTracing enables debug tracing.
+//
+// This complies with the [TracingLexer] interface.
+func (r *RegexLexer) SetTracing(trace bool) {
+	r.trace = trace
+}
+
 // A CompiledRule is a Rule with a pre-compiled regex.
 //
 // Note that regular expressions are lazily compiled on first use of the lexer.
@@ -185,6 +195,7 @@ func (l *LexerState) Get(key interface{}) interface{} {
 
 // Iterator returns the next Token from the lexer.
 func (l *LexerState) Iterator() Token { // nolint: gocognit
+	trace := json.NewEncoder(os.Stderr)
 	end := len(l.Text)
 	if l.newlineAdded {
 		end--
@@ -205,14 +216,33 @@ func (l *LexerState) Iterator() Token { // nolint: gocognit
 		}
 
 		l.State = l.Stack[len(l.Stack)-1]
-		if l.Lexer.trace {
-			fmt.Fprintf(os.Stderr, "%s: pos=%d, text=%q\n", l.State, l.Pos, string(l.Text[l.Pos:]))
-		}
 		selectedRule, ok := l.Rules[l.State]
 		if !ok {
 			panic("unknown state " + l.State)
 		}
+		var start time.Time
+		if l.Lexer.trace {
+			start = time.Now()
+		}
 		ruleIndex, rule, groups, namedGroups := matchRules(l.Text, l.Pos, selectedRule)
+		if l.Lexer.trace {
+			var length int
+			if groups != nil {
+				length = len(groups[0])
+			} else {
+				length = -1
+			}
+			_ = trace.Encode(Trace{ //nolint
+				Lexer:   l.Lexer.config.Name,
+				State:   l.State,
+				Rule:    ruleIndex,
+				Pattern: rule.Pattern,
+				Pos:     l.Pos,
+				Length:  length,
+				Elapsed: float64(time.Since(start)) / float64(time.Millisecond),
+			})
+			// fmt.Fprintf(os.Stderr, "%s: pos=%d, text=%q, elapsed=%s\n", l.State, l.Pos, string(l.Text[l.Pos:]), time.Since(start))
+		}
 		// No match.
 		if groups == nil {
 			// From Pygments :\