FEAT: Enable whitelist for compiling specific files

[mmcky]

It would be useful to have the ability to parse certain files only through the use of a whitelist (environment variable). The whitelist could then be used by the extension to skip writing a lecture not in the list during the Builder phase.

One use case is CI to check / compile specific lectures that hopefully can be inferred from the git environment.

We will need to keep in mind references and links if we only build some lecture files. I think the full doctree would be parsed and links resolved prior to our Builder phase. If this is not the case we can then implement an execution whitelist feature instead and build the whole lecture set as that doesn't take much time.

[mmcky]

Compiling specific files can be don using an environmental variable FILES= using Makefile or through the jupinx --files tool.