fix(codegen): apply reserved word escaping to union shape in Json serializer

[smilkuri]

Issue

Internal JS-6273

Description

Apply reservedWords.escape() to union shape names when generating the visitor pattern code to avoid TypeScript compilation errors when union names conflict with reserved words like 'Parameters'.

Testing

Locally

yarn build:all

 Tasks:    516 successful, 516 total
Cached:    42 cached, 516 total
  Time:    15m45.413s 

    ...Finishing writing to cache...                                                                                                                                                                                                                           WARNING  no output files found for task @aws-sdk/aws-middleware-test#build. Please check your `outputs` key in `turbo.json`

Checklist

• [n/a] If the PR is a feature, add integration tests (*.integ.spec.ts).
• [n/a] If you wrote E2E tests, are they resilient to concurrent I/O?
• [n/a] If adding new public functions, did you add the @public tag and enable doc generation on the package?

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