fix: jypytext not found and update vim health methods

[gogamid]

Fix: jypytext not available on macOS Sonoma 14.5

Problem: jypytext was not found on macOS Sonoma 14.5 using global package, virtual environment, or Python host via vim.g.

Solution: Using a table in vim.fn.system instead of a string fixes the issue.

Changes:

• Changed string to table in vim.fn.system.
• Additionally updated vim.health methods.

[gogamid]

#23

[hahuang65]

@GCBallesteros any chance we could get this merged? The checkhealth fix would be very nice.

[GCBallesteros]

@hahuang65 Sorry for the radio silence. Will review and merge soon.

[tjni]

With this change, in init.lua, the calls to vim.fn.shellescape now need to be removed.

[idrilirdi]

Actually, for me the change to a table for cmd breaks the functionality for some reason. The call to jupytext is: jupytext 'file.ipynb' --output='file.md' --to=md:markdown but this returns 1 with a "no such file or directory" for 'file.ipynb'. The exact same command in the shell works, so keeping cmd as a string makes it work as it's then called through the shell