Improve add_rule.py and add_plugin.py scripts (#3725)

scripts/_utils.py

@@ -13,5 +13,12 @@ def pascal_case(linter_name: str) -> str:
     return "".join(word.title() for word in linter_name.split("-"))
 
 
+def snake_case(name: str) -> str:
+    """Convert from PascalCase to snake_case."""
+    return "".join(
+        f"_{word.lower()}" if word.isupper() else word for word in name
+    ).lstrip("_")
+
+
 def get_indent(line: str) -> str:
     return re.match(r"^\s*", line).group()  # type: ignore[union-attr]