nvim: Fix scratchpad functionality

`:ScratchPad` command has been fixed and its functionality slightly
extended:

`:ScratchPad` will replace the current buffer with an empty scratchpad,
with the filetype defined in `g:scratchpad_ft` (or the buffer-scope
equivalent).

`:ScratchPad!` will create a new split and open the empty scratchpad
there.

Additionally, a single argument can be passed to the command with the
name of the filetype that the scratchpad should contain, e.g.
`:Scratchpad! golang`

Most of the plugin process has been shifted to lua and only a single
command mapping remains as vimscript for now.

nvim/.config/nvim/spell/en.utf-8.add

@@ -72,3 +72,7 @@ dialogue
 Uber
 precarity
 financialization
+Walmart
+Wrocław
+Poznań
+Taylorization