The default commenting character is now '#', considering the superior
amount of files that use it for comments compared to the previous
C-style comment characters.
The logic now also prioritize the opening commenting characters if they
contain a colon, to be able to use selection commenting on markdown
files.
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.
