Allow custom rules to be provided with raw contents / via an fs.FS

[jamietanna]

When using the Go API for regal, it's possible to set custom rules by path.

However, this only works when the files are available on disk.

For instance, if you wanted to wrap regal as part of a separate binary, we'd want to package up the files in the binary with an embed.FS.

When an embed.FS is used, we'd need to unwrap the directory and copy it to the local file system to use.

It'd be convenient if we could either:

• Provide a map of filename-to-contents
• Provide an fs.FS (which satisfies embed.FS)

This would allow more flexibility for different use-cases.

[anderseknert]

Agreed, that sounds like a good addition.