gh-152033: Use a leading category as the INFO charset prefix

_get_charset_prefix() did not recognize a leading bare CATEGORY opcode,
so a pattern starting with a category escape (such as ``\d``) lost its
SRE_INFO_CHARSET prefix and search() could no longer skip non-matching
start positions -- a regression relative to the IN-wrapped form.  Handle
CATEGORY there too, which restores the charset-prefix optimization and
makes search() of category-prefixed patterns up to ~1.9x faster.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/re/_compiler.py

@@ -495,7 +495,7 @@ def _get_charset_prefix(pattern, flags):
         if iscased and iscased(av):
             return None
         return [(op, av)]
-    elif op is CHARSET:
+    elif op is CATEGORY:
         return [(op, av)]
     elif op is BRANCH:
         charset = []