Port UnionConverter to STJ re #43

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The bulk of the commits in this PR derive from https://github.com/jet/FsCodec/pull/43 by @nickdarvey

In particular, the entire implementation and test suite was already complete, but was parked (and inadvertently closed, hence its in this PR).

The later commits effect minor cleanups:

• [x] uses [ugly] #ifdefs to make clear the commonalities (and, most importantly, the minor differences) between the System.Text.Json and Newtonsoft.Json implementations wrt Pickling and JsonIsomorphism
• [x] unifying the UnionConverterTests to test the STJ and JNK variants with a common test suite
• [x] cherry-picking nested unions support from #52
• [x] fix 3 failing tests signifying incomplete handling of nested unions from ☝️

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This has a set of failing test cases that need to be fixed.

The impl should end up being low double digit lines of code, but parking for someone with the time to do some reading of STJ source and/or fiddling about to handle the more complex cases cleanly!

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Woohoo I found the problem/solution (some tidying and/or extending of test suite still to be done, but seems this is on track for finally being released) cc @NickDarvey