Contrast compliant syntax highlighting: V2

Created by: umpox

Description

Follow up to https://github.com/sourcegraph/sourcegraph/pull/37385 and feedback we have received.

This PR:

• Further updates contrast contrast. We have decided to prioritise the typical viewing experience (e.g. not diff-added|remove or selected-line). We will address those areas in a future high-contrast theme. This gives us the extra flexibility to make our code highlighting readable and compliant at the same time.
• Adds semantic colors. Right now, this is mainly so we can iterate on this fast and understand what we're changing. In the future we should aim to use the colors everywhere, instead of hl-blue etc, as that is not brittle to change.

These changes are feature-flagged. See #37385 for more information

Example before

Example after

Test plan

Local testing and ARC Toolkit audits

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.