[css-images] Adding logical values for directions in linear-gradient()

[yarusome]

Current syntax for linear-gradient() is:

linear-gradient() = linear-gradient(
  [ <angle> | to <side-or-corner> ]? || <color-interpolation-method>,
  <color-stop-list>
<side-or-corner> = [left | right] || [top | bottom]

And the acceptable values for <side-or-corner> make adaptation to different writing modes difficult, e.g. using <hr> with a non-zero block size and a linear gradient as its background image. Therefore, it seems useful to make <side-or-corner> additionally accept [block-start | block-end] || [inline-start | inline-end].

Relevant section in the spec: https://w3c.github.io/csswg-drafts/css-images-4/#linear-gradients

[cdoublev]

You might be interested by #1724.