Fix for names starting with ints

Author: Gobot1234

src/betterproto/casing.py

@@ -135,4 +135,8 @@ def lowercase_first(value: str) -> str:
 
 def sanitize_name(value: str) -> str:
     # https://www.python.org/dev/peps/pep-0008/#descriptive-naming-styles
-    return f"{value}_" if keyword.iskeyword(value) else value
+    if keyword.iskeyword(value):
+        return f"{value}_"
+    if value[0].isdigit():
+        return f"_{value}"
+    return value