settings: Change "remove trailing whitespace on save" to default false for Markdown (zed-industries#41658)

Closes #ISSUE: reported on X by user. 

Release Notes:

- Made it so that the default value for the "remove trailing whitespace
on save" setting in Markdown is false, to fix cases where the removed
trailing whitespace had syntactic meaning

Author: katie-z-geer

assets/settings/default.json

@@ -1763,6 +1763,7 @@
     "Markdown": {
       "format_on_save": "off",
       "use_on_type_format": false,
+      "remove_trailing_whitespace_on_save": false,
       "allow_rewrap": "anywhere",
       "soft_wrap": "editor_width",
       "prettier": {