Speedup formatting

[machitgarha]

This is a WIP for speeding up markdown restyling by doing partial restyling where possible instead of restyling the whole document every time. Targets #142.

Read commit comments for more context.

[machitgarha]

This should be ready. Please test it.

For me, its speedup is really noticeable for large files with many formattings.

[toolstack]

Thanks, I'll give it a go and see how it works.

[toolstack]

Ok, done some basic testing and looks good so far 👍

Will do a more thorough review over the next few days.