Maps Block: Add Geolocation button to find current location

#### Is your feature request related to a problem? Please describe.
The search and location selection isn't perfect, and finding your "current" location isn't super easy.

#### Describe the solution you'd like
I think having a "You are Here" button that uses the [Geolocation API](https://developer.mozilla.org/en-US/docs/Web/API/Geolocation_API) would be beneficial and easy to implement.

I'm happy to look into this some and try my hand at a PR if you think this would be useful.

Locate the Maps Block entry point and read the MDN Geolocation API documentation linked in the issue. Confirm how the block currently handles search and location selection, then define completion as a visible current-location button that uses the browser location and updates the map appropriately.

Written by the indexing model from the issue text.