Edit fenced code blocks indented

The indentation is removed before the editing and replace at the end
Better fenced code block identification
Edit single line code block

Search first for a fenced code block, if not found go and edit a range
of lines even if the range is a single line. This let me remove the mode
argument to the command making it more directly usable
Create code block if unable to find one

MdEditCodeBlock will create a code block if unable to find one to edit.
The definition of unable to find one aka if a typed code block is not
found and the range is a single empty line. The file type is asked, by
default is markdown, if it's markdown then the code is directly inserted
otherwise a fenced code block is added around the code
Edit code block mappings in insert mode