Support core.commentString configuration

[abhinav]

The core.commentChar configuration was superseded by core.commentString
which allows for a multi-character string instead of a single character.

This commit adds support for core.commentString handling.
The existing comment_char method was renamed to comment_string
which internally falls back to core.commentChar
if core.commentString is unset.

[abhinav]

This change is part of the following stack:

• Support core.commentString configuration #99 ◀

_{Change managed by git-spice.}