syntax-highlighter: Use sourcegraph/syntect, not slimsag/syntect.

Created by: varungandhi-src

@tjdevries can you double-check that there's no behavior change with this? I tested a few languages visually.

Test plan

The tests in sourcegraph/syntect are passing. Visually inspected the highlighting for different languages like JavaScript, Ruby and Markdown with this change -- didn't see any regression.