fix: no underscores after numbers when converting to snake case

Author: YodaEmbedding

src/betterproto/casing.py

@@ -8,11 +8,11 @@
 
 # Optionally capitalized word.
 # language=PythonRegExp
-WORD = "[A-Z]*[a-z]*[0-9]*"
+WORD = "[A-Z]*([a-z]|([0-9][A-Z]*))*"
 
 # Uppercase word, not followed by lowercase letters.
 # language=PythonRegExp
-WORD_UPPER = "[A-Z]+(?![a-z])[0-9]*"
+WORD_UPPER = "[A-Z]+(?![a-z])[A-Z0-9]*"
 
 
 def safe_snake_case(value: str) -> str: