[outtmpl] Allow `\n` in replacements and default.

Fixes: https://github.com/yt-dlp/yt-dlp/issues/6808#issuecomment-1510055357
Fixes: https://github.com/yt-dlp/yt-dlp/issues/6808#issuecomment-1510363645

test/test_YoutubeDL.py

@@ -822,6 +822,7 @@ class TestYoutubeDL(unittest.TestCase):
         test('%(title&foo|baz)s.bar', 'baz.bar')
         test('%(x,id&foo|baz)s.bar', 'foo.bar')
         test('%(x,title&foo|baz)s.bar', 'baz.bar')
+        test('%(title&\n|)s', '\n')
 
         # Laziness
         def gen():

yt_dlp/YoutubeDL.py

@@ -1156,7 +1156,7 @@ class YoutubeDL:
         }
         MATH_FIELD_RE = rf'(?:{FIELD_RE}|-?{NUMBER_RE})'
         MATH_OPERATORS_RE = r'(?:%s)' % '|'.join(map(re.escape, MATH_FUNCTIONS.keys()))
-        INTERNAL_FORMAT_RE = re.compile(rf'''(?x)
+        INTERNAL_FORMAT_RE = re.compile(rf'''(?xs)
             (?P<negate>-)?
             (?P<fields>{FIELD_RE})
             (?P<maths>(?:{MATH_OPERATORS_RE}{MATH_FIELD_RE})*)

yt_dlp/options.py

@@ -243,7 +243,7 @@ def create_parser():
         if multiple_keys:
             allowed_keys = fr'({allowed_keys})(,({allowed_keys}))*'
         mobj = re.match(
-            fr'(?i)(?P<keys>{allowed_keys}){delimiter}(?P<val>.*)$',
+            fr'(?is)(?P<keys>{allowed_keys}){delimiter}(?P<val>.*)$',
             value[0] if multiple_args else value)
         if mobj is not None:
             keys, val = mobj.group('keys').split(','), mobj.group('val')