On the instances with altfile this was already the case, but this makes it more obvious.

Closes #3673

When an invalid colorscheme is selected from the user configuration
file, the editor doesn't report any errors at startup unless the
`fail` command is used.

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

Automatic reparsing of %sh{...}, while convenient in many cases,
can be surprising as well, and can lead to security problems:

'echo %sh{ printf "foo\necho bar" }' runs 'echo foo', then 'echo bar'.
we make this danger explicit, and we fix the 'nop %sh{...}' pattern.

To reparse %sh{...} strings, they can be passed to evaluate-commands,
which has been fixed to work in every cases where %sh{...} reparsing
was used..

Style tweaks, and remove check for parameter count as this is already
done by Kakoune itself (the command is specified to take 1 parameter)

Makes it easier for users who want to locate their kakrc file, and
does not require to go through shell expansion to get it as
"${XDG_CONFIG_DIR:-${HOME}/.config}/kak"

Fixes #1740