Normalize labelhash for ENSRainbow Heal request

[tk-o]

@lightwalker-eth suggested

Suggest to define some utility function for normalizeLabelhash that takes the provided labelhash and then "normalizes" it into the format expected by the the ENSRainbow heal API.

Of course, normalizeLabelhash should throw an error if the provided labelhash cannot be normalized. This error should flow through the heal function being defined here. In the JSDoc comment on the heal function suggest documenting a @throws for the case that labelhash is not a valid labelhash.