Use WSGI SCRIPT_NAME (request.script_root) for file urls instead of requests_pathname_prefix

Is your feature request related to a problem? Please describe.
Dash currently requires setting requests_pathname_prefix when hosting under a sub-path for generating file URLs, despite the fact that the WSGI spec and Flask offer a provision for this by the way of the SCRIPT_NAME variable (Available in Flask in request.script_root) and used by Flask utilities like url_for when in request context by default.

There is also routes_pathname_prefix which is effected by SCRIPT_NAME adding to the confusion.

The is obviously all very confusing.

Describe the solution you'd like
Use SCRIPT_NAME via request.script_root to generate file URLs, removing the need to change requests_pathname_prefix probably deprecating it. Backwards compatibility concerns apply.

Describe alternatives you've considered
Keep having to manually set requests_pathname_prefix in each app, which also requires modifying the app source file today.

Additional context
SCRIPT_NAME & PATH_INFO needs to be set correctly by the combination of WSGI server and proxy. The way to do so varies.

Start by tracing how Dash uses requests_pathname_prefix and routes_pathname_prefix when generating file URLs, then compare that behavior with Flask's request.script_root from WSGI SCRIPT_NAME. Verify file URLs when the app is hosted under a sub-path and determine how existing requests_pathname_prefix behavior can remain compatible or be deprecated.