Added simple text coloring, especially for twinx plots ylabels.

[theDigitalGuy]

I wanted something like this

import matplotlib.pyplot as plt
import mpl_typst.as_default

color1="blue"
color2="tab:olive"
sigA = [0, 1, 2, 3, 10]
sigB =[0, -1, -2, -3, -10]

fig, axes = plt.subplots(1,1)
axes.plot(sigA, color=color1)
axes.set_ylabel("Signal A", color=color1)

ax2 = axes.twinx()
ax2.plot(sigB, color=color2)
ax2.set_ylabel("Signal B", color=color2)
fig.savefig("test1.typ")

to produce

but the draw_text function in backend.py didn't respect the colors.

This pullrequest should be a simple fix. Not entirely sure how to name the new color variable in the resulting typst file. Chose tcolor for now, to not confuse it with the stroke color (not used yet), but also didn't want do wast more bytes on the additional "ext" of textcolor, but feel free to change that.

If we are add it we could probably also add support for the alpha channel of the text color like backend_pgf.py. I think the typst rgb color function directly supports the alpha value as an additional forth percent value.