[text-spacing-trim] The initial prototype

This patch is the initial prototype implementation of the
`text-spacing-trim` property, for a proof of the concept of
adding the `halt` features to part of a run. See the "Code
Flow" section of the design doc[1].

Because the initial value of the property applies the kerning,
and the implementation is not complete, the runtime flag is
changed to none, and tests are done in the virtual test suite.

[1] https://docs.google.com/document/d/146Bupkg3nrNALL3bm8UElRd0vsLVH5807xubsnrthDw/edit#bookmark=id.qdxlxbjhmail

Bug: 1463891
Change-Id: I56e72356ce129c396fff44792f4370089f6aac26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4819066
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1191234}