bug(markdown) Applying a blockquote to a header line will remove the header hash marks and indented code

[yellis]

Describe the bug
Applying a blockquote to a header line will remove the header hash marks. This will happen regardless of what (if anything) is highlighted on the line at the time when the blockquote function is activated. Works fine in RT mode.

To Reproduce
Steps to reproduce the behavior:

1. MD mode. Type in # this is a header
2. While on the line, ctrl-q or use the blockquote button
3. The header # disappears

Will also remove indented code formatting. Ie, this:

    Triage result      Questions Avg Views 
    ------------------ --------- --------- 
    Looks Good         11379     76        
    Should Be Improved 9825      66        
    Unsalvageable      4299      48     

Becomes this:

> Triage result      Questions Avg Views 
> 
> Looks Good         11379     76        
> Should Be Improved 9825      66        
> Unsalvageable      4299      48 

Expected behavior
Blockquote application in MD mode should not remove header hashes

Screenshots

Desktop (please complete the following information):
Windows 11 Chrome 103

Additional context

• https://meta.stackexchange.com/a/380318/51
• https://meta.stackexchange.com/a/363589/51

[b-kelly]

Same root issue as #70, closing this issue to keep the list tidy.