inset: Add aliases for R and J and fix two examples

[seisman]

Description of proposed changes

Fix two inset examples as pointed out in #1930 (comment).

Closes #1930.

[willschlitzer]

Good improvement to this tutorial

[seisman]

I mark the PR as a draft due to the concern/issue mentioned in #1930 (comment)

[seisman]

As explained in GenericMappingTools/gmt#6751 (comment), a better solution is specifying -R and -J in inset begin so that inset can determine the inset width and height automatically. I think this is an intuitive solution and should be recommended, so I have updated the two examples accoordingly.

Unfortunately, -R and -J are not aliased in PyGMT v0.6.1, so this PR also adds the aliases for -R and -J.

Some TODOs:

• [ ] Update https://github.com/GenericMappingTools/pygmt/blob/main/examples/gallery/embellishments/inset.py (not necessary because width=height for the map in the inset)
• Update https://github.com/GenericMappingTools/pygmt/blob/main/examples/gallery/embellishments/inset_rectangle_region.py
• Remove the inline example for inset (https://github.com/GenericMappingTools/pygmt/blob/8ddd11ce7314e1dba34fbfa428204b54e6fd15c3/pygmt/src/inset.py) because we already have a very similar example in the gallery and the gallery image is shown at the bottom of the API documentation page. [Fixed in fcee181] [I've added the inline example back based on the comment in https://github.com/Add inline code examples in plotting module docstrings #1957#issuecomment-1155592855]

Please let me know what you think before I start to work on them.