nvim: Move to treesitter and lspsaga

Added lspsaga to enhance some of the lsp actions I can do - mappings are
mostly the same as before, pretty much all lsp actions can be invoked by
g<mnemonic> (e.g. `gr` for rename, `gh` for symbol help, `gd` for go
definition, `K` for help hover, `gc` for code completion and some more).
`[e` and `]e` move between lsp diagnostic errors.

Switched out many syntax highlighting plugins for treesitter, which is
an experiment for now, but I would love to keep it like this if it works
out.

nvim/.config/nvim/plugin/completion-nvim.vim

@@ -64,7 +64,7 @@ let g:completion_chain_complete_list = {
       \    { 'mode': '<c-n>'}
       \],
       \ 'default': [
-      \    { 'complete_items': [ 'lsp', 'snippet' ]},
+      \    { 'complete_items': [ 'ts', 'lsp', 'snippet' ]},
       \    { 'complete_items': [ 'buffers' ] },
       \    { 'complete_items': [ 'path' ], 'triggered_only': ['/'] },
       \    { 'mode': '<c-p>'},