modification methods on `Coordinates`

[keewis]

For ease of use, this implements a few additional methods on Coordinates:

• rename_dims
• rename_vars
• drop_vars
• coords1 | coords2 as an alias of coords1.merge(coords2)

These currently forward to the implementation in Dataset, but will be the natural place after refactoring Coordinates to not wrap a Dataset.

---

• [x] towards #10314
• [x] Tests added
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst

cc @benbovy, @scottyhq