Add verbatim parsing of lstlisting environments

[j-moeller]

The pull request fixes the parsing of the lstlisting environment which should be similar to the verbatim environment. Since I am not deeply familiar with the code base, most diff is basically copy-pasted from the verbatim environment (without the additional parameterization for the macro).

I added a test case which contains source code with a single opening curly bracket. If the lstlisting environment is not parsed verbatim, this corrupts the parse tree (every node after the lstlisting environment becomes a child of the environment).

[phfaist]

Hi @j-moeller, thanks for your pull request! Supporting lstlisting environments is a great idea. Here are a couple thoughts:

• I wonder though whether there's a reason for a new parser class, instead of using the existing verbatim argument parser class? (It might have to be adapted to also know about lstlisting environments.) I'd especially like to avoid duplicating code because it makes it much harder to maintain.
• On the other hand, there might be good reasons to keep a separate ParsedLstListingArgs class (in case in the future pylatexenc is able to extract more information from an lstlisting environment, maybe individual lines, etc.).
• It looks like the lstlisting environment accepts an optional argument. It would be great if pylatexenc supported that, too!

Concretely, what I suggest would be:

• add some features to the VerbatimArgsParser class: (a) if verbatim_arg_type == 'verbatim-environment' make its constructor accept an additional argument verbatim_environment_name defaulting to "verbatim" (but which could be set to "lstlisting") which sets the name of the environment that contains the verbatim content; (b) make the constructor accept another argument, say env_argspec, providing a specification of the macro arguments that the begin-environment itself accepts (this way we can say that the lstlisting environment accepts an optional argument); (c) make the constructor accept another argument, which should be the class object that will store the parsed argument(s) (by default ParsedVerbatimArgs). The class should derive from ParsedVerbatimArgs and its constructor should accept suitable keyword arguments (TBD) (maybe to include the verbatim environment name, additional arguments, etc.).
• Then the lstlisting environment can be declared as: EnvironmentSpec('lstlisting', args_parser=VerbatimArgsParser(verbatim_arg_type='verbatim-environment', verbatim_environment_name='lstlisting', env_argspec='[', parsed_args_class=ParsedLstListingArgs))

Thanks again for contributing and let me know if I'm missing anything!