There is no such functionality at the moment.

But there are ways to achieve this:

1. export the workspace by using the databricks cli. It will download all the notebooks etc, then you can perform a search with your IDE or ...
2. use Repos. The workbooks are sent to git (Github, Azure Devops,...) and there you can go to your files and perform a search there.

The 2nd option is ofc a better approach.