refactor filer types

[ryanatkn]

This renames some of the Filer and Compiler types. It also changes the Filer's compiled file type to wrap compilations instead of extending them. This provides a cleaner separation between the compiler's return value and all of the subsequent metadata that we add (there will be more in the future), at the cost of some duplication of properties.

This is in preparation for the postprocess step which makes the compiled file's contents potentially diverge from the original compilation's, and by storing the original reference, we retain the un-postprocessed version. We need to postprocess files to update their module paths to support Svelte and other usecases. I currently like the idea of isolating the Compiler to not touch module paths, just like the TypeScript and Svelte compilers.