fix: prevent error when using a proc in asset_host (close #202)

[ElMassimo]

Description 📖

This pull request fixes #202 by ignoring asset_host in Rails when configured with a Proc.

Background 📜

Usually, the asset host will be used as the base for Vite, which is a good default, especially when asset_host is configured to target a CDN.

Rails allows to use a Proc to dynamically configure asset_host, allowing to return different values depending on the source, the URL of an asset.

Since there's no "right" value to pass to Vite when asset_host is a Proc, it's better to ignore it, which in most cases would still allow assets to be correctly resolved.

In the few cases where this is not desired, assetHost can be explicitly configured in config/vite.json, config/vite.rb, or via environment variables.