ENH: initial attempts at code summary layer, sphinx domain

[klauer]

Changes in no particular order

• Reworked how Meta instances are transferred over from lark to blark; add explicit dataclass attributes for them
• Trying out a layer that summarizes transformed code for easier access ("tell me the function blocks, their declared variables/methods/actions, and where to find them")
• Building a sphinx Domain on top of it (looking at sphinx-julia, built-in Python support for inspiration/ideas how to do it)
• Rework some weird code flow from the original blark parse CLI
• Some test cleanup
• More test coverage for global var declarations
• Made mypy a lot happier for blark.transform (never 100% happy of course)
• Add UNION support (Closes #14 )
• Additional grammar fixes (need to document more of the differences noted when running TwinCAT. As to be expected, blark is more permissive in some cases and too restrictive in others)

Documentation?

So far, just function blocks + methods:

Generated by way of pointing to the project in Sphinx conf.py:

.. bk:functionblock:: FB_TempSensor

• Cross-referencing works with function block names, variable/method names.
• Functions, programs, etc. will be easy enough to add when I get time
• No intention of making sphinx or docutils a required runtime dependency to blark

Things which I don't have a clue how to approach just yet:

• Cross-referencing API docs from source code listings
• And some form of syntax highlighting
• Autodoc (project -> auto-documented pages)

[klauer]

Despite the above, I'm going to redo this entirely working with docutils.nodes and sphinx addnodes (removing Jinja from the equation). It's a more standard way of doing things, though I'm not finding much documentation on how those nodes should be structured, so it may take a bit.

[klauer]

There's still a lot left to do here, but the structure is OK enough as-is to merge for now.

There will be follow-ups down the line at some point. One day entire PLC projects' API will be easily documentable in sphinx, but today is not that day...