Site Migrations: Attempt to auto-detect non-WP platforms

### What

Based on the report at https://github.com/Automattic/wp-calypso/issues/98645, it would be nice if we could detect other platforms on this screen in the flow:

To reproduce:

1. Create a site from `/start`
2. Select "Import or migrate" from the goals screen
3. Input your non-WP-based URL
4. You'll be redirected to the Import screen where you have to select your platform manually (even if your platform's name is part of your URL):

### Why

This would make for one less step in the importer flow, less resistance for users.

### How

_No response_

Reproduce the flow from /start by choosing “Import or migrate” and entering a non-WordPress URL, then inspect the Import screen and the existing report at issue 98645. Determine where platform selection is handled and define done as automatically detecting a supported non-WordPress platform from the URL so the manual selection step is skipped.

Written by the indexing model from the issue text.