Add key mapping to exit terminal mode via ESC

I was unable to exit the terminal mode using the default shortcut
C-\ followed by C-n when using 'de' as keyboard layout. Maybe because of
the required additional modifier key to access backspace. This caused
me to always 'exit' the shell in order to get back to the edit buffers.

I would assume that more people are affected when using a different
keyboard layout than 'us' and would benefit from a simpler default.

init.lua

@@ -342,6 +342,9 @@ vim.keymap.set('n', ']d', vim.diagnostic.goto_next, { desc = 'Go to next diagnos
 vim.keymap.set('n', '<leader>e', vim.diagnostic.open_float, { desc = 'Open floating diagnostic message' })
 vim.keymap.set('n', '<leader>q', vim.diagnostic.setloclist, { desc = 'Open diagnostics list' })
 
+-- Exit terminal mode by simply pressing ESC
+vim.keymap.set('t', '<ESC>', '<C-\\><C-n>', { noremap = true })
+
 -- [[ Highlight on yank ]]
 -- See `:help vim.highlight.on_yank()`
 local highlight_group = vim.api.nvim_create_augroup('YankHighlight', { clear = true })