Refactor neovim config (#2)

Changes include:

- [x] Use init.lua instead of init.vim
   - Some keybindings are still to be converted
- [x] Use packer as a package manager instead of vim-plug
- [ ] Use built-in lsp instead of coc.nvim
   - [x] Set up language servers
   - [x] Completion
   - [x] Formatting (previously coc-prettier, now neoformat)
   - [ ] Snippets
- [ ] Replace fzf with telescope.nvim
- [x] Implement treesitter syntax highlighting
   - More info: https://github.com/nvim-treesitter/nvim-treesitter

Note that this requires neovim nightly until 0.5 is released

Reviewed-on: #2
Co-authored-by: Marko Korhonen <marko@korhonen.cc>
Co-committed-by: Marko Korhonen <marko@korhonen.cc>

home/.config/nvim/lua/keybinds.lua

@@ -0,0 +1,27 @@
+local function map(mode, lhs, rhs, opts)
+    local options = {noremap = true}
+    if opts then options = vim.tbl_extend('force', options, opts) end
+    vim.api.nvim_set_keymap(mode, lhs, rhs, options)
+end
+
+-- Open/close tree browser
+map('n', '<C-Tab>', '<cmd>NvimTreeToggle<CR>')
+
+-- Telescope
+map('n', '<C-f>', '<cmd>Telescope find_files find_command=fd,-Ht,f<CR>')
+map('n', '<C-g>', '<cmd>Telescope live_grep<CR>')
+
+-- Completion
+-- Navigate completions with tab and shift tab
+map('i', '<S-Tab>', 'pumvisible() ? "\\<C-p>" : "\\<Tab>"', {expr = true})
+map('i', '<Tab>', 'pumvisible() ? "\\<C-n>" : "\\<Tab>"', {expr = true})
+
+-- Navigate between buffers
+map('n', '<C-N>', ':bn<CR>', {silent = true})
+map('n', '<C-B>', ':bp<CR>', {silent = true})
+
+-- Navigate between splits
+map('n', '<C-H>', '<C-W><C-H>')
+map('n', '<C-J>', '<C-W><C-J>')
+map('n', '<C-K>', '<C-W><C-K>')
+map('n', '<C-L>', '<C-W><C-L>')