Issue with HTML tags inside MFM block

[null2264]

Issues

• <small>Small text</small> works but $[blur <small>Small text</small>] isn't, probably caused by mfm-parser seems to be caused by Earmark.
• Things started with a double underscore (__) is parsed incorrectly.

All of these issues could be fixed by using md instead of Earmark since they support custom block.

Tasks

• Migrate to https://github.com/am-kantox/md
  Currently FastSanitize screw up the end result of md when MFM is involved FastSanitize (Or lexbor to be specific) follows HTML specs very strictly, <center> won't work inside <span>. May need to make a custom handler for MFM inside md, or make an in-house markdown (MFM) parser from scratch.
  
  In Misskey (or at least in Firefish) <center> is actually <div> with text-align: center CSS. <center> HTML tag inside fn block ($[... ]) will be escaped, other HTML tags seem to be fine? the "rule" is very loose and inconsistent in MFM.