Fix BMesh typing (#179)

[Road-hog123]

• BMesh sequences were missing generic base class, so generic arguments and return values were not changing with subscription.
• Additionally BMesh sequences were being given methods they don't have, and BMIter was not being used as iterable return type.
• BMLayerCollection's get() method now has a return type.
• Added __getitem__(), __setitem__() and __delitem__() to BMVert, BMEdge, BMLoop and BMFace.*

Fixes #179

*Presently these methods get/set Any—to fix this BMLayerItem needs to be made generic (trivial), but that generic type needs to come from BMLayerCollection which also needs to be made generic, which involves modding all of its methods and uses (non-trivial), and I don't think it's currently possible to bind that generic type to BMLayerItem (but it might be after #161's work on PEP 695 is implemented?).

[nutti]

@Road-hog123

Thank you for your contribution! If this PR is ready, please remove WIP status.

[Road-hog123]

Together with #122374 - PyAPI Doc: improve BMesh typing most issues with BMesh should be fixed. 👍

[nutti]

@Road-hog123

Thank you for your effort to fix the issue!