Add the "proximity sensor" block

- add the `obstacle distance" as an input number block
- add the "when there is an obstacle" as a when block (triiger clock)

No files, tests, or entry points are named. Locate the repository's existing block definitions and related sensor or trigger tests, then use them to determine where these additions belong. Done means the proximity sensor exposes an “obstacle distance” input number block and a “when there is an obstacle” trigger block.

Written by the indexing model from the issue text.