Add support for extension based configuration.

[milanc]

Description

This would allow to start using the extension with existing projects without need to go over each file and add it to compiler config files.
Main reason for this is using the extension with scss for Blazor components, where I would like to avoid adding each razor.scss file to config.

[milanc]

Hi @failwyn, I've created PR at the same time as this issue.
When you get some free time please let me know if it makes sense.

[mstancombe]

This would be great..
@failwyn Can this one be merged in?

[failwyn]

I just haven't been able to find the time to thoroughly test it; I'll try to merge it and run it on my development machine for a while to see if any issues arise.