support transforming only a given range/visual selection

[superhawk610]

The current implementation transforms the entire buffer. It would be nice to allow transforming only a particular range, either explicitly provided via :h :command-range (:-2,+2AsciiBlockify) or by calling with a visual selection (:'<,'>AsciiBlockify). This will require modifying the AsciiBlockifycommand atplugin/ascii_blocks.vim` to accept a range but default to the entire file

- command AsciiBlockify :call ascii_blocks#transform_selection()
+ command -range=% AsciiBlockify :call ascii_blocks#transform_selection()

and/or updating the blockify_current_buf function in lua/ascii_blocks.lua to optionally accept start/end row/column arguments, something like

function M.blockify_current_buf(start_row, start_col, end_row, end_col)
  start_row, start_col = start_row or 0, start_col or 0
  end_row, end_col = end_row or -1, end_col or -1 -- not sure -1 works but you'd want to get the
                                                  -- the last row/column in the buffer
  -- (snip) --
end

You can get the row/column of the '< and '> marks using vim.fn.getpos("'<"), see this thread.