Add syntax highlighting of Pkl code (#1385)

This adds syntax highlighting of Pkl code!

It adds highlighting for:

* Stack frames within error messages
* CLI REPL (highlights as you type, highlights error output)
* Power assertions (coming in https://github.com/apple/pkl/pull/1384)

This uses the lexer for highlighting. It will highlight strings,
numbers, keywords, but doesn't understand how to highlight nodes like
types, function params, etc.
The reason for this is because a single line of code by itself may not
be grammatically valid.

pkl-parser/src/main/java/org/pkl/parser/Lexer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -65,6 +65,14 @@ public class Lexer {
     return new String(source, sCursor, cursor - sCursor);
   }
 
+  public int getStartCursor() {
+    return sCursor;
+  }
+
+  public int getCursor() {
+    return cursor;
+  }
+
   public char[] getSource() {
     return source;
   }