Figure.plot: Rename "fill_between" to "between_fill" (alias for -M)

[yvonnefroehlich]

Description of proposed changes

Rename fill_between to beween_fill for _M to be consistent with the other *_fill parameters.
Extends: #3169

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[yvonnefroehlich]

In PR #3169, fill_between was added as an alias for -M of Figure.plot. This is consistent with the method used by matplotlib (https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.fill_between.html). However, to be consistent with other parameter names, it should be between_fill, e.g., positive_fill and negative_fill of Figure.wiggle. See also #3169 (comment). @GenericMappingTools/pygmt-maintainers What do people prefer:

• 🎉 Keep fill_between
• 🚀 Change to between_fill

[seisman]

One thing to note is that in parameters such as positive_fill and negative_fill, fill is a noun, whereas in fill_between, it is more like a verb.