JetBrains: add simple diff view

Created by: vdavid

Fixes https://github.com/sourcegraph/sourcegraph/issues/35550 in a simple way.

All changes listed:

• Shows a raw diff in the preview for diff searches.
• Adds an "Open on Sourcegraph" link to the type:diff, type:commit and also the type:repo search result types, so now we have no dead ends.
• Fixes a bug that we previously overlooked with @philipp-spiess
• Implemented a simple save/load to/from localStorage for the bridge to simulate the real behavior better and perhaps make testing easier.

Test plan

Video where I demo all the above: https://www.loom.com/share/3bc07f04cabd48ef88ccc7a8654871b1 (sorry, I blather a lot in the demo because I realized some bugs while doing the demo. The bugs are irrelevant to these changes, though: I confirmed this by testing on main after the vid.)

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.