This commit properly produces backslash characters within double quote
strings instead of hoping the shell will not recognize the escape
sequence that they form with the following character. Use the proper
POSIX function declaration form.
The uncommenting logic now also ignores trailing newline characters,
which shortens the amount of operations needed to uncomment a selection.
Initialising the `comment_line_chars` and `comment_selection_chars` variables
in language support scripts created a hard dependency of those scripts
to `commenting.kak`, which would create errors when this script was not
loaded, e.g. when running tests.
