fix: add support for non-root base-href

[holzgeist]

Description

if a web-app is deployed using --base-href=/path/to/deployed/flutter/app during build, the URLs starting with ./assets won't work anymore. Using assetManager will properly resolve them. It uses assetBase from the loader config specified here

NB :warning:

I only tested the url.startsWith('assets/') code path as it's the only one used in the library. It should work the same for ./ though