I keep having 3+ PR's in at the same time and always deal with crazy
conflicts because everything in the `ext` library is together in one
file.
This PR moves each category of extension into its own file, and
separates the `vfs` functionality in Core into the `ext/dynamic` module,
so that it can be more easily separated from wasm (or non feature =
"fs") targets to prevent build issues.
The only semantic changes made in this PR is the feature gating of vfs,
the rest is simply organizing and cleaning up imports.
Was unsure if `vfs` should be a feature on the `core` side too, or to
just enable it with the `fs` feature which seemed reasonable, as that
was already the current behavior. But let me know if we want it entirely
behind it's own feature.
Reviewed-by: Pere Diaz Bou <pere-altea@homail.com>
Closes#1124
