Change default Markdown soft_wrap to "bounded" (#27205)

- Follow-up to: https://github.com/zed-industries/zed/pull/26247

Previously with defaults meant that Markdown would softwrap even if you
had available window space (soft_wrap occurred at default
`preferred_line_length` of 80).

Release Notes:

- Changed Markdown default to soft_wrap at window width instead of
preferred_line_length

assets/settings/default.json

@@ -1228,7 +1228,7 @@
       "format_on_save": "off",
       "use_on_type_format": false,
       "allow_rewrap": "anywhere",
-      "soft_wrap": "bounded",
+      "soft_wrap": "editor_width",
       "prettier": {
         "allowed": true
       }