RFC Initial pygmt.param.Pen class for specifying pen attributes

[weiji14]

Description of proposed changes

Start a pygmt.param.Pen convenience class that generates a string which can be passed into GMT's pen (-W) parameter. The Pen class is structured to be under a 'param' folder. Added three basic unit tests to ensure various width/color/style combinations
will be output correctly. Note that this is still a basic implementation, and does not include advanced + modifiers as in https://docs.generic-mapping-tools.org/latest/plot.html#w.

Preview API docs at https://pygmt-git-params-pen-gmt.vercel.app/api/generated/pygmt.param.Pen.html

Example code:

import pygmt

# Thick, purple, dashed-dot-dashed border line around some text
pen = pygmt.param.Pen(width="thick", color="purple", style="-.-")
print(pen)
# thick,purple,-.-

fig = pygmt.Figure()
fig.text(x=1, y=1, region=[0, 2, 0, 2], pen=pen, text=pen)
fig.savefig("text_pen.png")
fig.show()

produces:

References:

• https://docs.generic-mapping-tools.org/latest/cookbook/features.html#specifying-pen-attributes

Fixes #1173, part of #1082

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 adding new functionality, add an example to docstrings or tutorials.

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[weiji14]

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