Support code sections within structured text documents (Markdown, etc)

[grwlf]

Hi. I would like to use LSC for navigating in code sections of structured text documents like LaTeX, Markdown, etc. Consider a Latex example:

\begin{pythoncode}
import numpy as np
import tensorflow
\end{pythoncode}

Some arbitrary text between code sections

\begin{pythoncode}
print(np.sin(3.14))
\end{pythoncode} 

Following #260 I was able to register Python language server for whole .tex document by calling

call RegisterLanguageServer('tex', {'command': 'pylsp',  'languageId':'python' })

There are a number of drawbacks though: First, this enables only one language per tex-file (here - Python), second and the most important, the LSC probably sends the whole .tex to the language server so one would consider himself lucky if it is able to parse the Python snippets out of it. It would be much better to pass only the related code sections to the server.

If implemented, this feature would be a good match with the "offline" Jupyter-like VIM plugin called Litrepl I am currently working on.