Conceal link modifier

[mrossinek]

This adds support for concealing the link modifier, :. Since tree-sitter will not allow us to do concealing natively we will have to rely on our trusty Vim syntax regions defined via regexes anyways.

Actual text

Concealed

Up for discussion

Right now, I implemented the link modifier regex to only require being suff- and prefixed with another markup modifier on the opening and closing link modifiers, respectively. However, this somewhat contradicts the current spec which requires a non-whitespace character in front of the opening modifier. However, I find that is a limitation because it will prevent e.g. the following: This is :*im*:possible!
I believe, that this is a not uncommon scenario so I am open for suggestions on how to rephrase the spec accordingly. I placed a temporary TODO comment at the relevant position of the markdown file.
As of right now, the spec does not contain a similar limitation for the closing :.

PS: I also did some minor touch-ups on the other conceal elements