Line numbers on online validator

[johnmorrell]

When using the online validator (https://validator.fhir.org) it would be useful if line numbers were visible alongside the code (similar to a GitHub diff), so reported errors can easily be identified.

[dotasek]

This is a feature we've wanted to include for some time.

Line numbers are currently available when you use the Upload Resources tab, as a current workaround.

To implement this feature, the UI for the Enter Resource needs to be redesigned to work with Ace editor. Since Validation is too slow for 'live' editing, we'll need two modes: one for editing, and one for viewing validation results.

I was envisioning something similar to GitHub's Write / Preview tabs but am leery of nesting too many tabs in the UI, as we already have Enter Resource / Upload Resources