Introduces the gr-syntax-layer element. This element works as an
annotation layer that is configured with the diff and asynchronously
computes/applies syntax for the diff.
Introduces a custom build of Highlight.js which gr-syntax-layer makes
use of. Building the script is documented in
scripts/vendor/highlight/building.md.
The layer is connected to the annotation pipeline in gr-diff-builder as
the lowest layer and syntax processing is triggered only after a diff
has been completely rendered.
A number of styles are added to the gr-diff element for syntax markers.
Tests added for gr-syntax-layer.
Bug: Issue 3916
Change-Id: Ic33e40f4fe39dfce1a62de133cfaf32be5e3f25a
650c529276