Add linting functionality.

[TheResplandor]

Add linting functionality into vim, and then add linters for python and cpp.

might use:

• ale: can be integrated with coc.
• some coc extension.

[TheResplandor]

built-in lsp adds linting functionality. Also i no longer use linters except for linting docs (formatters are better). Need to: 1) add linters to cpp docs and other languages. 2) check null-ls.nvim for adding external linters.