feat(support): add json encode/decode to array and string utilities

[xHeaven]

Closes #1388

[brendt]

Perfect! One thing I wonder about is whether we can automatically determine mutability based on the class

So ImmutableString > ImmutableArray, MutableArray > MutableString, etc.

@innocenzi do we already have a detection method for this in place? I actually don't think we do?

If we don't, then let's not overcomplicate this PR. I'd remove the mutable parameter, always return the Immutable version (I believe this is consistent with the rest of the support helpers) and solve the mutability problem as a separate PR.

[innocenzi]

I don't think we should have a mutable option, we can always call toMutableArray and toMutableString on the returned instance

[innocenzi]

Also, how about dropping json from the method name? arr()->encode() and str()->decode()?

If not, I would prefer encodeJson and decodeJson instead of the other way around

[brendt]

Let's

• use encodeJson and decodeJson
• drop the mutable option and always return the immutable variant

[xHeaven]

@brendt @innocenzi take a look 👀

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