Merge Task into Task2

[thilp]

TIP/transformer#10 “Arbitrarily nested scenarios” solved TIP/docs#330 “Introduce generic multi-level nested scenarios in Transformer” by introducing the transformer.python module, which represents Python code as a syntax tree (i.e. a tree of objects, each representing a syntactic element like "class" or "assignment").

transformer.python.OpaqueBlock does not represent a regular Python element but encapsulates a string and "pretends" it is a normal syntax tree. It helps by not forcing the introduction of the syntax tree to instantly propagate everywhere in Transformer: without OpaqueBlock, Task and all plugins would have had to be updated in TIP/transformer#10.

However, eventually this usage of OpaqueBlock should go away: plugins should be updated to use the more powerful & testable framework of transformer.python too. Once plugins are updated, Task can be merged into the Task2 proxy. At that point, the only remaining usage of OpaqueBlock should be for cases not yet covered by the rest of transformer.python (like the raise statement today).

• Define new types of plugins that interact with Task2 and/or the syntax tree instead of Task and its strings → Design better plugin contracts for the Syntax Tree Framework #10
• Convert all existing plugins into "new type" plugins.
• Merge Task into Task2, keeping the name "Task".
• Update our plugin documentation.

ℹ️ Issue imported from TIP/docs#395