Add a simple markdown formatter

[nemethf]

Add a built-in markdown formatter derived from org-mode, but prefer
the third-party markdown-mode even if it's not available. Hence, keep
the current behavior unless the user customizes the new variable
`eglot-markup-formatter'.

• eglot.el (eglot-markup-formatter): New defcustom.
  (eglot-client-capabilities, eglot--format-markup): Use it.
  (eglot-simple-markdown-mode): New defun.

[astoff]

I think the following mistakes would happen quite often:

• asterisks as bullet points -> org headings
• markdown headings -> org comment

[nemethf]

I think the following mistakes would happen quite often:

• asterisks as bullet points -> org headings
• markdown headings -> org comment

Headings and bullet points are rare in my use-case, and the only practical consequence of those mistakes for me is unusual font-locking. The PR allows me to understand code examples more quickly as they colored. (Another mistakes are unclickable links, but I don't care about that either.)

Maybe this is not for everyone, but I don't propose to be used as a default.

[joaotavora]

Felicián Németh ***@***.***> writes:

Add a built-in markdown formatter derived from org-mode, but prefer the third-party markdown-mode even if it's not available. Hence, keep the current behavior unless the user customizes the new variable `eglot-markup-formatter'.

Hi Felicián, sorry for taking so long to look at this. I think this idea of a built-in markdown formatter is on the right track except that I think that this should be added to Emacs's lisp/emacs-lisp/eldoc.el instead. The idea is for Eglot to provide -- inasmuch as possible -- un-rendered docs to eldoc.el, so that eldoc-display-functions can do the rendering based on the actual viewport they are rendering to. This should help solve Emasc bug#61072. I'll propose this soon in either that bug or a new one, and I'll keep you in the loop. João