It allows plugins to create generic terminal using the user's preferred windowing system
For example, it can be used to run fzf, gdb or simply a shell.
* 'new' commands are refactored to simply use the 'terminal' one
* style and docstrings has been unified
* all windowing systems go through "sh -c" for consistency purposes, even if unnecessary
This commit also introduces a regression in that I decided that the best way to
avoid overly long and confusing names was to rename the current shell-*
switches to script-*, and have the shell command completion be
shell-completion.
renamed script-{completion,candidates} to shell-script-*
Updated docs with new switch names
Added -shell-completion switch to x11-repl and kitty-repl
