Add support for FHIR validator's new watch mode

[vadi2]

See hapifhir/org.hl7.fhir.core#1311 - this (should) reduce validation time because the validator won't need to load all of the necessary packages at startup every single time you need to validate, and instead just once at the start of the bindary.

Relevant thread to read up on: https://chat.fhir.org/#narrow/stream/179239-tooling/topic/Validator.20CLI.20watch.20mode