Fix get_capability typing overloads

[alcarney]

Description (e.g. "Related to ...", etc.)

By trying to fix #592 I realised that the typing overrides we generate for get_capability don't make much sense.

• First there was the issue where Sequence[str] was reported as str.

• However, the script also tried to unpack types like Union[DefinitionOptions, bool]

  I'd appreciate a second opinion, but I don't think this is correct?

  Since the types in the union have different "shapes" you can't guarantee that definition_provider.work_done_progress exists so wouldn't the safer option be to stop at definition_provider to encourage the caller to handle each type separately?

  Assuming the answer to this is "yes", I've tweaked the script to only unpack Optional[T] capabilities

As a result, many overloads have been deleted.

This PR also tidies up the generate_code.py script and migrates away from deprecated typing syntax

Closes #592

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• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
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• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly

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[alcarney]

Thanks! :)