Introduce JsonUtilsNew as documented in #5875 (#6355)

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

.github/actions/spell-check/expect/expect.txt

@@ -2255,6 +2255,7 @@ targetnametoken
 targetver
 taskbar
 tbar
+TBase
 tbc
 tbi
 Tbl
@@ -2297,6 +2298,7 @@ testtestabc
 testtesttesttesttest
 TEXCOORD
 texel
+TExpected
 textattribute
 TEXTATTRIBUTEID
 Textbox
@@ -2321,6 +2323,7 @@ tilunittests
 Timeline
 titlebar
 TITLEISLINKNAME
+TJson
 tl
 TLEN
 Tlg
@@ -2343,6 +2346,7 @@ tooltip
 TOPDOWNDIB
 TOPLEFT
 TOPRIGHT
+TOpt
 tosign
 touchpad
 towlower
@@ -2385,6 +2389,7 @@ Txtev
 typechecked
 typechecking
 typedef
+typeid
 typeinfo
 typelib
 typename