Add LineCollection plot

[Illviljan]

This adds a line plotter based on LineCollections, called .lines.

I wanted to replace darray.plot() with using LineCollection instead. But unfortunately due to how many cases are supported (and tested in xarray) darray.plot() will continue using plt.plot.

xref: #4820 #5622

[Illviljan]

Scatter vs. Lines:

ds = xr.tutorial.scatter_example_dataset(seed=42)
hue_ = "y"
x_ = "y"
size_="y"
z_ = "z"

fig = plt.figure()
ax = fig.add_subplot(1, 2, 1, projection='3d')
ds.A.sel(w="one").plot.lines(x=x_, z=z_, hue=hue_, linewidth=size_, ax=ax)
ax = fig.add_subplot(1, 2, 2, projection='3d')
ds.A.sel(w="one").plot.scatter(x=x_, z=z_, hue=hue_, markersize=size_, ax=ax)