Refactor: Mark internal methods as private

[ThisIsMissEm]

This cleans up the documentation a bit, here's the existing:

And the new:

Whilst one could argue this is a breaking change, by making app private, it didn't currently work correctly for registering additional routes due to the issues described in #8

[ThisIsMissEm]

I'm unsure if it actually makes sense to expose the methods formatLabel, labelIsSigned and signLabel, since the signing of labels is internal to the labeler service. Removing these from the public API would allow a refactoring of the types that would make the documentation clearer:

We could have a Label type, where CreateLabelData and SavedLabel is:

// Assumes Label has `sig: Bytes; src: Did; uri: Did | CanonicalResourceUri;`
type CreateLabelData = MakeOptional<Omit<Label, "$type" | "ver" | "sig"> & { src: string, uri: string }, "cts" | "src">
type SavedLabel = Label & { id: number; }

Which would get us down to just three exported types for the public API for labels, which would be more understandable from a public API documentation perspective.