CodeMirror blob: override browser's search to trigger CM search

Created by: olafurpg

Towards #40724.

Test plan

Manually tested this change by triggering Cmd+F without an active focus on the blob view.

Note that Cmd+F still triggers the browser's "Find" feature if the search bar is focused, and it's automatically focused when reloading a blob view page. This issue can be addressed separately.

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.