hether the
said line should be removed or left in place. If the shebang line
contains a path (even a single /) then it is assumed to be a real
shebang line and left alone. However, if no path is given
(e.i.: `#!python` or `:::python`) then it is assumed to be a mock shebang
for language identification of a code fragment and removed from the
code block prior to processing for code highlighting. When a mock
shebang (e.i: `#!python`) is found, line numbering is turned on. When
colons are found in place of a shebang (e.i.: `:::python`), line
numbering is left in the current state - off by default.

Also parses optional list of highlight lines, like:

    :::python hl_lines="1 3"
r