Now that we have a comprehensive JSON diagnostic structure, we can use
it in the `validate -json` output instead of the inline version. Note
that this changes the output of `validate -json` in two ways:
1. We fix some off-by-one errors caused by zero-width highlight ranges.
This aligns the JSON diagnostic output with the text output seen by
most Terraform users, so I consider this a bug fix.
2. We add the `snippet` field to the JSON diagnostics where available.
This is purely additive and is permitted under our JSON format
stability guarantees.
Also uncomment and fix some tests which had been skipped for a couple of
years. Those validate cases work now!
Note that these test cases and the JSON output are not especially
minimized, making them snapshot/golden tests. The output looks correct
at time of writing, and we don't expect to change validate significantly
any time soon, but if we do there will be some churn here.
Alisdair McDiarmid