Read the [JsonUtils Spec] for more details.
This pull request introduces the next version of JsonUtils. It is in a
separate file for ease of review and testing.
JsonUtilsNew will be renamed in a subsequent commit that rewrites our
JSON deserializers.
### Implementer's Notes
I went with telescoping exceptions for the key parsing code, because
it's totally possible that you can be five keys deep and encounter a
type error. This lets us encode information about all failures in the
chain instead of just the topmost one.
The original JsonUtilsNew code changed to use `decay` everywhere because
the tests wouldn't compile. We want to treat `GetValue<const guid>` _the
same as_ `GetValue<guid>`, and this lets us do so. `decay` is awesome.
I've been developing this with a shim that redirects `JsonUtils.h` to
`JsonUtilsNew.h`. I am not comfortable deleting the original until we've
moved off of it, and that _will_ be the subject of a followup PR.
## Validation Steps Performed
So many tests.
[JsonUtils Spec]: https://github.com/microsoft/terminal/blob/master/doc/cascadia/Json-Utility-API.md
Refs #2550