We've had a bunch of inconsistency between `channel_bag` and `channelbag` in the
code base. After discussion with @dr.sybren, we decided to standardize on
`channelbag` and also rename the camelcase `ChannelBag` to `Channelbag` to be
consistent with that.
This PR implements those changes.
Note that the reason we standardized on `channelbag` rather than `channel_bag`
is because it makes things clearer when stringing multiple terms together in
type and function names. For example, in `channelbag_fcurves_move()` it makes
it clear that `channelbag` describes one thing, rather than `channel` and `bag`
being two separate things.
No functional changes intended.
Pull Request: https://projects.blender.org/blender/blender/pulls/130988
089c3cd85c