This commit updates JsonUtilsNew to support winrt
`Windows::Foundation::IReference<T>` as an option type, and cleans up the
optional support code by removing the optional overload on
`GetValue(...)`. Instead of using an overload with a partial
specialization, we're using a constexpr if with a type trait to
determine option-type-ness.
In addition, Carlos reported an issue with deriving from `FlagMapper`
(itself templated) and referring to the base type's members without
fully qualifying them. To make derivation easier, `EnumMapper` and
`FlagMapper` now provide `BaseEnumMapper` and `BaseFlagMapper` type
aliases.
I've taken the opportunity to add a `winrt::hstring` conversion
trait.
Lastly, in casual use, I found out that I'd written the til::color
converter wrong: it supports color strings of length 7 (`#rrggbb`) and
length 4 (`#rgb`). I mistyped (and failed to test) support for 4-length
color strings by pretending they were only 3 characters long.
## References
Merged JsonUtils changes from #6004 and #6590.
## PR Checklist
* [x] Unblocks aforementioned PRs
* [x] cla
* [x] Tests added/passed
* [x] Documentation N/A
* [x] Schema N/A
* [x] Kid tested, mother approved.
89c4ebaafe