Better typing for enums and flags

[almarklein]

Closes #720

Context

Enums in Python are not great, and we want/need simple strings to be passed for arguments that represent an enum. We want to support both of these:

wgpu.gpu.request_adapter_async(power_preference=wgpu.PowerPreference.high_performance)
wgpu.gpu.request_adapter_async(power_preference="high-performance")

The first is the "full-enum" approach, which can actually be made to work with the builtin enums.Enum. The latter is a lighter, and arguably more readable approach, also used by WebGPU in JS.

In terms of autocompletion, we have that working for the first approach. This PR adds it also for the latter (in IDE's that are smart enough to understand typing.Literal).

Requirements

• In an interactive session, see the options by typing >> wgpu.PowerPreference and hitting enter.
• In an interactive session, get autocompletion by typing >> wgpu.PowerPreference.hi.
• Static type checking.
• Static autocompletion by typing (power_preference=".
• Generation of clear API docs.

Implementation

Implementing this is apparently not trivial: python/typing#781

In this PR I went with the approach where for each enum and flag, we have two flavours, one for typing and one to act as the public flag/enum object.

To stick with the power-preference example:

# In enums.py

PowerPreferenceEnum = Literal["low-power", "high-performance"] | str

class PowerPreference(Enum):
    low_power = "low-power"
    high_performance = "high-performance"

# In _classes.py

async def request_adapter_async(
        self,
        *,
        feature_level: str = "core",
        power_preference: enums.PowerPreferenceEnum = None,
        force_fallback_adapter: bool = False,
        canvas=None,
    ) -> GPUAdapter:
        """ ...
        Arguments:
            ...
            power_preference (PowerPreference): "high-performance" or "low-power".
            ...
        """
        ...

The PowerPreferenceEnum is only used for typing and is not public. Notice that the docstring uses the public PowerPreference, so the Sphinx docs will make that a link to the docs for that enum.

[almarklein]

Relies on #737 for CI to pass.

[almarklein]

Ready. I have some more typing improvements I want to do, but I'll make new prs for that.