Implement InterpolableFontPalette

Implement custom InterpolableValue for font-palette property. Represents
interpolation of mix value.

Explainer: https://docs.google.com/document/d/1XMTrKH003KBOes6hxzI-3E7LTwp5YwFC-rnzoFpFrfw/edit?usp=sharing

Bug: 1400620
Change-Id: Ia4e84f57acd3346b7108bee9aab3e2cb9b982b6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4487158


Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Munira Tursunova <moonira@google.com>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148542}