regex: add support for RegexFlag class

stubs/regex/regex/_regex_core.pyi

@@ -1,8 +1,45 @@
+import enum
 from typing import AnyStr
 
 class error(Exception):
     def __init__(self, message: str, pattern: AnyStr | None = ..., pos: int | None = ...) -> None: ...
 
+class RegexFlag(enum.IntFlag):
+    A: int
+    ASCII: int
+    B: int
+    BESTMATCH: int
+    D: int
+    DEBUG: int
+    E: int
+    ENHANCEMATCH: int
+    F: int
+    FULLCASE: int
+    I: int
+    IGNORECASE: int
+    L: int
+    LOCALE: int
+    M: int
+    MULTILINE: int
+    P: int
+    POSIX: int
+    R: int
+    REVERSE: int
+    T: int
+    TEMPLATE: int
+    S: int
+    DOTALL: int
+    U: int
+    UNICODE: int
+    V0: int
+    VERSION0: int
+    V1: int
+    VERSION1: int
+    W: int
+    WORD: int
+    X: int
+    VERBOSE: int
+
 A: int
 ASCII: int
 B: int