Add macro for subtypes lowering with additional `subtypekey` field

[sairus7]

Not sure if this API covers all cases, maybe some other key types should be added (now its only Symbol).

closes #93 and https://github.com/quinnj/JSON3.jl/issues/12

[quinnj]

Ah, looks like we have some CI failures; mind taking a look @sairus7?

[sairus7]

Now it should be working.

[codecov[bot]]

Codecov Report

Patch coverage: 91.66% and project coverage change: +0.82 :tada:

Comparison is base (4dbad5b) 74.48% compared to head (bff19d2) 75.31%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #94 +/- ## ========================================== + Coverage 74.48% 75.31% +0.82% ========================================== Files 2 2 Lines 388 401 +13 ========================================== + Hits 289 302 +13 Misses 99 99 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaData/StructTypes.jl/pull/94?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaData) | Coverage Δ | | |---|---|---| | [src/macros.jl](https://app.codecov.io/gh/JuliaData/StructTypes.jl/pull/94?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaData#diff-c3JjL21hY3Jvcy5qbA==) | `95.65% <91.66%> (+4.74%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/JuliaData/StructTypes.jl/pull/94/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaData)

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