Allow handling arbitrary Map keys as JSON object properties

[Tarmil]

Since .NET 6, the handling of dictionary keys can be customized using JsonConverter.Write/ReadAsPropertyName. Our Map converter should try use these methods in the same way, and only fall back to array-of-pairs in JSON if it can't.

[Tarmil]

It's actually not really possible to choose the format based on whether the key type's converter supports {Read|Write}AsPropertyName. There isn't a flag that tells whether they're supported; just these two methods, that will either succeed or fail when called.

So a better option would be to follow what @cmeeren suggests in #152 and allow choosing the format in JsonFSharpOptions.

• A version, corresponding to the current default, where Maps whose keys are string and single-case-union-of-string are serialized as objects, and other keys are serialized as array-of-pairs.
• A new version where all keys are handled with {Read|Write}AsPropertyName. This will therefore always use JSON objects, but only support key types that support {Read|Write}AsPropertyName. This includes types such as int, Guid, DateTime, etc. With #161 implemented, this also includes single-case-union-of-string. In other words, the set of key types supported by this format is a superset of the key types that are serialized as JSON objects in the current version.
• Maybe even a version using array-of-pairs regardless of the key type, because it's easy to do and may be appealing to some users for both supporting all key types and being consistent across key types.