Introduce nvim-tmux-navigator

Also integrate with my custom setup using tmux window options to
communicate between nvim and tmux for which panes contain nvim
processes.

lua/plugins/tmux.lua

@@ -0,0 +1,35 @@
+return {
+  "alexghergh/nvim-tmux-navigation",
+  config = function()
+    local plugin = require('nvim-tmux-navigation')
+    plugin.setup({
+      disable_when_zoomed = true,
+    })
+
+    -- Integrate tmux navigation flag
+    local navigation_flag =
+      '@vim' .. vim.fn.substitute(vim.env.TMUX_PANE, '%', '\\%', 'g')
+    local function set_navigation_flag()
+      vim.fn.system('tmux set-window-option ' .. navigation_flag .. ' 1')
+    end
+    local function unset_navigation_flag()
+      vim.fn.system('tmux set-window-option -u ' .. navigation_flag)
+    end
+
+    -- [Un]set tmux window option to detect when to change pane.
+    set_navigation_flag()
+    local augroup = vim.api.nvim_create_augroup('tmux', { clear = true })
+    vim.api.nvim_create_autocmd('FocusGained', {
+      pattern = '*', group = augroup, callback = set_navigation_flag,
+    })
+    vim.api.nvim_create_autocmd('VimLeave', {
+      pattern = '*', group = augroup, callback = unset_navigation_flag,
+    })
+
+    -- Map nativation bindings
+    vim.keymap.set('n', "<C-h>", plugin.NvimTmuxNavigateLeft)
+    vim.keymap.set('n', "<C-j>", plugin.NvimTmuxNavigateDown)
+    vim.keymap.set('n', "<C-k>", plugin.NvimTmuxNavigateUp)
+    vim.keymap.set('n', "<C-l>", plugin.NvimTmuxNavigateRight)
+  end
+}