Implement Transpose for XTensorVariables

[ricardoV94]

@AllenDowney I cherry-picked your changes from #1427, I squashed all the transpose related changes into a single one, and split unrelated changes into other 2 commits.

I pushed a fourth commit with changes that I'm listing here, and I would appreciate if you could in turn review them:

1. Moved the missing_dims out of the Op since this is not really a parameter that affects how the Op behaves at runtime, and is therefore useless. I put the behavior in the helper that calls the Transpose Op instead. The Op will expect dims to be correctly specified and raise if not.
2. Merged the helper expand_ellipsis into the make_node since that's the only place it's used and it's not insanely complex.
3. Put the 2 new transpose tests closer to the original one, and simplified the last test, no point (imo) in testing 2 functions for x.T
4. Tweaked error messages to match what xarray returns.

[AllenDowney]

@ricardoV94 Your changes look good to me (and Cursor).

[ricardoV94]

@ricardoV94 Your changes look good to me (and Cursor).

AIs are not very adversarial by default, but good to know. All merge it then