Figure.fill_between: Add parameter 'legend_pen' to draw legend entries as lines

[seisman]

Implement -M's +r modifier.

Note the upstream bug at GenericMappingTools/gmt#9070.

Address #4678.

Preview: https://pygmt-dev--4698.org.readthedocs.build/en/4698/api/generated/pygmt.Figure.fill_between.html

import numpy as np
import pygmt
x = np.linspace(0, 2 * np.pi, 200)
fig = pygmt.Figure()
fig.basemap(region=[0, 2 * np.pi, -1.2, 1.2], projection="X10c/4c", frame=True)
fig.fill_between(
    x=x,
    y=np.sin(2 * x),
    y2=np.sin(3 * x),
    fill="lightblue",
    pen="1p,blue",
    fill2="lightred",
    pen2="1p,red",
    label="y=y1(x)",
    label2="y=y2(x)",
)
fig.legend()

fig.shift_origin(xshift="w+2c")
fig.basemap(region=[0, 2 * np.pi, -1.2, 1.2], projection="X10c/4c", frame=True)
fig.fill_between(
    x=x,
    y=np.sin(2 * x),
    y2=np.sin(3 * x),
    fill="lightblue",
    pen="1p,blue",
    fill2="lightred",
    pen2="1p,red",
    label="y=y1(x)",
    label2="y=y2(x)",
    legend_pen=True,
)
fig.legend()
fig.show()