syn: Fix IDL named enum variant field being snake_case

[acheroncrypto]

Problem

Named enum variant fields are in snake_case as opposed to every other field being in camelCase in the IDL.

This has been addressed in the IDL build implementation(#2011) but the issue still persists with the parse method.

Summary of Changes

• Convert the named enum variant fields to camelCase
• Add a test case that uses an enum variant named field with multiple words(in snake_case)

It would be great if we could get rid of the case conversion in IDL but that would also have its own downsides(breaking change).

Closes #1443.

[vercel]

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