Autostrip functionality

[flowchartsman]

Essentially this library is a union wrapper over an embedded FS, so it's currently necessary to call http.StripPrefix, just as you would when using http.FS to wrap an fs.FS, however, swaggerui is intended to be a convenient "out-of-the-box" integration, and having a mandatory stdlib call for every invocation is kind of a pain. With a little thought, it might be possible to have the handler inspect the route path and automatically determine the prefix and serve up the correct file(s).

Given that the wrapped FS does not actually reference the filesystem, there shouldn't be any problems with attempted path traversals.

If this is something people want to see for V1, please vote via 👍/👎 and feel free to discuss your opinion below (esp. in the case of 👎)