Remove nested JSON.stringify from props serialization

[belluzj]

Changes

• Only call JSON.stringify once when serializing props for an island
• Match this change when deserializing props for an island
• This fixes a quadratic increase in quote escaping when using nested data, see Exponential bloat in prop serialization #7978

Testing

I added a test that the kind of nested data from #7978 results in a normal-sized serialized text.

I didn't add more tests in this branch because I'm not comfortable changing the code organization in the project but I committed to another branch how I tested the implementation, basically by adding roundtrip tests to the file serialize.test.js. However I had to move the deserialization to the same file as the serialization, which I'm not sure fits the project structure. See daltonmaag@a513407

Docs

I believe this change should be transparent to users so I'm not sure it needs documenting?

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[natemoo-re]

This looks good to me as a hotfix for astro@2.x. We'll want to revisit this approach for astro@3.x to avoid the &quote; encoding issue entirely.