[Feature Request] Add style options to set line and point opacities

[neodescis]

Is your feature request related to a problem? Please describe.
Currently it is possible to style a line or point's colors, but not its opacity (or opacities, if you want to include the point outlines).

Describe your proposed idea for the solution to this problem
I would like to be able to vary line and point opacity just like I can with polygon fill.

Describe alternatives you've considered
It appears that I can force a color of "transparent" (or probably "rgba(0, 0, 0, 0)") by bypassing the HexColor type checking, at least in MapLibre.