[BUG] TypeScript generation changes property name

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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output? => See Description
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Description

Hi, first of all, thank you for this wonderful tool :-)

My issue:
_

When generating a model for TypeScript, the variable/key "_" is not transformed correctly.

Example YAML Schema:

    FooType:
      required:
        - _
      type: object
      properties:
        _:
          type: string
      description: The FooType
      discriminator:
        propertyName: _
        mapping:
          Bar: '#/components/schemas/Bar'

Generated model in Typescript (actual):

export interface FooType { 
    _u: string;
}

Generated model in Typescript (expected):

export interface FooType { 
    _?: string;
}

What works:
_

When renaming the variable "_" to something like "property", the generated model is expected and working properly. However, due to
backwards-compatibility issues, we can't simply rename this variable.

openapi-generator version

openapi-generator-cli version => 5.1.1

Suggest a fix

Do not change the variable name? ;-)

Start with the supplied FooType OpenAPI schema and reproduce the generated TypeScript model in openapi-generator-cli 5.1.1. Trace the TypeScript generator's property-name handling for the underscore key and compare it with the working output for a renamed property. Done means the underscore remains unchanged and the generated model matches the expected optional property declaration, with a regression check for this schema.