Allow stopping of URL detection at newlines via url_excluded_characters

Fixes #6122

kitty/options/definition.py

@@ -469,10 +469,15 @@ are still clickable.
     )
 
 opt('url_excluded_characters', '',
-    ctype='!url_excluded_characters',
+    option_type='python_string', ctype='!url_excluded_characters',
     long_text='''
 Additional characters to be disallowed from URLs, when detecting URLs under the
 mouse cursor. By default, all characters that are legal in URLs are allowed.
+Additionally, newlines are allowed (but stripped). This is to accommodate
+programs such as mutt that add hard line breaks even for continued lines.
+:code:`\\\\n` can be added to this option to disable this behavior. Special
+characters can be specified using backslash escapes, to specify a backslash use
+a double backslash.
 '''
     )
 

kitty/options/parse.py

@@ -3,8 +3,8 @@
 # isort: skip_file
 import typing
 from kitty.conf.utils import (
-    merge_dicts, positive_float, positive_int, to_bool, to_cmdline, to_color, to_color_or_none,
-    unit_float
+    merge_dicts, positive_float, positive_int, python_string, to_bool, to_cmdline, to_color,
+    to_color_or_none, unit_float
 )
 from kitty.options.utils import (
     action_alias, active_tab_title_template, allow_hyperlinks, bell_on_tab, box_drawing_scale,
@@ -1302,7 +1302,7 @@ class Parser:
         ans['url_color'] = to_color(val)
 
     def url_excluded_characters(self, val: str, ans: typing.Dict[str, typing.Any]) -> None:
-        ans['url_excluded_characters'] = str(val)
+        ans['url_excluded_characters'] = python_string(val)
 
     def url_prefixes(self, val: str, ans: typing.Dict[str, typing.Any]) -> None:
         ans['url_prefixes'] = url_prefixes(val)