Figure.meca: Add aliases for "L", "T", and "Fr"

[yvonnefroehlich]

Description of proposed changes

This PR aims to add aliases for pygmt.Figure.meca:

• L -> outline (Julia outline_pen)
• T -> nodal (Julia nodal)
• Fr -> labelbox (Julia Fr)

As related to this, also the docstrings for pen and offset are updated.

Preview: https://pygmt-dev--2546.org.readthedocs.build/en/2546/api/generated/pygmt.Figure.meca.html

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.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[yvonnefroehlich]

/format

[seisman]

Looks good to me.

[seisman]

Ping @GenericMappingTools/pygmt-maintainers to have another reviewer's approval.