This commit teaches `RgbColor::from_rgb_str` to support
colors in the form `hsl:235 100 50`, an HSL colorspace
color specification.
While banging my head on why my test wasn't passing, I realized
that this was producing 10 bpc color and the code to convert
those to RGB was incorrectly multiplying conversion terms!
refs: https://github.com/wez/wezterm/issues/1436
This commit introduces the `Dimension` type which allows specifying
a value in a variety of units; pixels, points, cells, percent.
`Dimension` needs contextual information to be evaluated as pixel
values, which makes resolving the value from the config slightly
more of a chore.
However, this type allows more flexible configurations that scale
with the font size and display dpi.
refs: #1124, #291, #195
