experiment: allow sideloading files from fs.FS

[srenatus]

1. add a layer of abstraction, fs.FS, into the code path for loading files
2. expose a new package global, virtualInput
3. setting virtualInput will load the provided fs.FS directly, and return an input
4. all calls to NewInput (all commands incl. LSP) will merge with the "virtual input", if provided

The reason for this: in EOPA, we've got a bunch of rego helper functions, and we'd like the LSP to be aware of them for code completion. This would seem like a decent approach to make it aware of them.

(The actual code completion UX still depends on #1145, because that is how the utility methods are advertised to be used.)

[srenatus]

What this helper did for us isn't much, so I've inlined it below, added the indirection to read from fs.FS.