Implement document highlights for search-based intel

Created by: olafurpg

Followup from https://sourcegraph.slack.com/archives/C0W2E592M/p1653291648908039

We currently only provide document highlights for Squirrel search-based intel

https://github.com/sourcegraph/code-intel-extensions/blob/a42e7d479aae09a83ee671a65c0876d7100d1f7c/template/src/search/providers.ts#L295

It would be nice if we had a fallback implementation that highlights all ranges matching the current token, similar to how VS Code does it by default.