boost regex tolerates non-escaped special characters, and escaped
non-special characters. Standardize on stricter syntax, where
special characters must be escaped, and non-special characters must
not.
The option is now used as a fallback when detection by extension fails. Some
scripts like `base/mail.kak` and `base/html.kak` still rely heavily on it.
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.
