Fix two auto-indent issues with Markdown and YAML (#20193)

Closes #13376
Closes #13338

Release Notes:

- Fixed unhelpful auto-indent suggestions in markdown.
- Added `auto_indent_on_paste` setting, which can be used on a
per-language basis, to configure whether indentation should be adjusted
when pasting. This setting is enabled by default for languages other
than YAML and Markdown.