Fix highlight "Code Search" navbar item

Created by: erzhtor

Closes https://github.com/sourcegraph/sourcegraph/issues/21373

• Add integration or unit tests

Test plan

Case 1 (search page):

• visit /search?q=test&patternType=regexp
• code search dropdown in the navigation should be highlighted

Case 2 (repo page):

• visit /github.com/sourcegraph-testing/zap
• code search dropdown in the navigation should be highlighted

Case 3 (repo file page):

• visit /github.com/sourcegraph-testing/zap/-/blob/README.md
• code search dropdown in the navigation should be highlighted

Case 3 (batch changes page):

• visit /batch-changes
• code search dropdown in the navigation should not be highlighted

App preview:

• Link
• Storybook

Check out the client app preview documentation to learn more.