Prepare v0.5.0 breaking changes

[bdistin]

This PR implements the following changes:

1. [BREAKING] The library no longer re-exports fs promises. This allows wider compatibility with newer versions of node's fs, without locking out lower versions by having to import/export newly added fs promise methods.
2. [FEATURE] The library now exports all types used for underlying nextra interfaces.
3. [BREAKING] The scan function is now updated to use the new openDir function from node v12.12.0's fs. This should be more performant than the previous implementation, however, instead of providing a Map<path, Stats>, a Map<path, Dirent> is returned. Dirent has most of the same methods as Stats (so your filter functions probably will work the same), but it is missing some finer details you get with Stats. Please note the new implementation will error if you try to scan on a file, unlike the previous implementation. This also fixes some weird behavior around the limit option.
4. [BREAKING] Because we are using something added in node v12.12.0, the minimum engine is now that version.
5. [N/A] Aliases are now exported from within the files they are aliasing.
6. [PATCH] This fixes a couple of functions that weren't exported correctly.

After some settling time, this version will be promoted to v1.0.0