Figure.meca: Add aliases for "Fa", "Fe", "Fg", "Ft", "Fp"

[yvonnefroehlich]

Description of proposed changes

This PR adds aliases for the GMT flags used for plotting the P and T axes for Figure.meca:

GMT flag	PyGMT alias
Fa	pt_axes
Fe	t_axisfill
Fg	p_axisfill
Ft	t_axispen
Fp	p_axispen

The docstrings are orientated on the upstream GMT documentation at https://docs.generic-mapping-tools.org/dev/supplements/seis/meca.html#f.

Preview: https://pygmt-dev--3526.org.readthedocs.build/en/3526/api/generated/pygmt.Figure.meca.html

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

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