add a way to prefer cached files over remote files

[donseba]

This pull request introduces a new method to handle caching and asset management for packages in the ImportMap structure. It also adds a supporting method in the Package structure to retrieve asset files. These changes aim to improve the handling of package assets and their integration into the application's import structure.

New functionality for caching and asset management:

• CacheOrFetch method in importmap.go:
  Added a new method to the ImportMap structure to check for cached and asset files for packages. If files are missing, it fetches or builds them as needed, ensuring the Structure.Imports map is updated with the correct asset paths. This method also includes logging for better traceability.

• Error handling improvements in importmap.go:
  Introduced the errors package to provide more descriptive error messages when cache or asset directories are not set.

Supporting method for asset retrieval:

• Assets method in library/package.go:
  Added a new method to the Package structure to list all asset files in a package's directory or retrieve a specific file. This method ensures proper error handling and returns detailed file information, including paths and types.