Add dedicated preview feature for East Asian Width (#4097)

Author: JelleZijlstra

src/black/lines.py

@@ -851,7 +851,7 @@ def is_line_short_enough(  # noqa: C901
     if not line_str:
         line_str = line_to_string(line)
 
-    width = str_width if mode.preview else len
+    width = str_width if Preview.respect_east_asian_width in mode else len
 
     if Preview.multiline_string_handling not in mode:
         return (

src/black/mode.py

@@ -195,6 +195,7 @@ class Preview(Enum):
     single_line_format_skip_with_multiple_comments = auto()
     long_case_block_line_splitting = auto()
     allow_form_feeds = auto()
+    respect_east_asian_width = auto()
 
 
 class Deprecated(UserWarning):