Change emum/flag implementation

[almarklein]

The current flags and enums work fine for interactive use (discoverable fields), but with static analysis (e.g. in VSCode) the fields are not visible.

Proposal:

• Copy the implementation from pygfx (https://github.com/pygfx/pygfx/pull/718)
• Add a config thingy to control the "context": the "wgpu" in <wgpu.BufferUsage ..>
• In pygfx re-use (instead of re-implement) the implementation from wgpu.

Related:

• Also look if something similar should be done for structs.
• #425