Modernize Vim completion and linting

[klappradla]

The ale and deoplete.vim setup is growing out of date and (especially the latter) causing massive startup-delays (spinning up own language servers, etc.). So maybe it's time to move over to something more modern - as in Lua based - and use Neovim's built in LSP 🤔

Some ideas:

• neovim/nvim-lspconfig has a lot of info about LSP setup (especially in the wiki)
• nvim-cmp seems a good completion solution to start with
• null-ls could replace ale for linters and even include formatters and code actions (e.g. for eslint)
• optional trouble.nvim for displaying diagnostics info
• optional mason.nvim for managing LSP and linter packages (probably overkill)

[klappradla]

In addition to that, maybe get rid of some "heavy" plugins and replace them with something from mini.nvim 🐥

[klappradla]

Done via #12