GH-121970: Combine custom Pygments lexers into a package (#121976)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Doc/tools/extensions/lexers/asdl_lexer.py

@@ -0,0 +1,42 @@
+from pygments.lexer import RegexLexer, bygroups, include
+from pygments.token import Comment, Keyword, Name, Operator, Punctuation, Text
+
+
+class ASDLLexer(RegexLexer):
+    name = "ASDL"
+    aliases = ["asdl"]
+    filenames = ["*.asdl"]
+    _name = r"([^\W\d]\w*)"
+    _text_ws = r"(\s*)"
+
+    tokens = {
+        "ws": [
+            (r"\n", Text),
+            (r"\s+", Text),
+            (r"--.*?$", Comment.Singleline),
+        ],
+        "root": [
+            include("ws"),
+            (
+                r"(module)" + _text_ws + _name,
+                bygroups(Keyword, Text, Name.Tag),
+            ),
+            (
+                r"(\w+)(\*\s|\?\s|\s)(\w+)",
+                bygroups(Name.Builtin.Pseudo, Operator, Name),
+            ),
+            # Keep in line with ``builtin_types`` from Parser/asdl.py.
+            # ASDL's 4 builtin types are
+            # constant, identifier, int, string
+            ('constant|identifier|int|string', Name.Builtin),
+            (r"attributes", Name.Builtin),
+            (
+                _name + _text_ws + "(=)",
+                bygroups(Name, Text, Operator),
+            ),
+            (_name, Name.Class),
+            (r"\|", Operator),
+            (r"{|}|\(|\)", Punctuation),
+            (r".", Text),
+        ],
+    }