Support multiline comments

[duna-akin]

Problem
GhostNotes currently only supports single-line comment syntax (e.g. #, //, --). Languages that use block/multiline comment delimiters (/* */, """ """, <!-- -->, {- -}, etc.) cannot have GhostNotes placed inside multiline comments, and those comment styles aren't supported as note markers at all.

This also means GhostNotes can't span multiple lines, a note is always a single line appended with # GN: some text.

Desired behavior

1. Block comment delimiters as note markers. Allow config entries like .html with <!-- , --> so users can write <!-- GN: note --> in HTML/XML files.
2. Multiline notes. Allow a note to span multiple lines, e.g.:

/* GN: This note
   spans multiple lines
   and is completely ignored */