[BUG] [Typescript] Add option to fallback to legacy enum naming

Bug Report Checklist

• 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?
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Description

The enum value generation has changed in 7.7.0 to add underscores to all generated enum values by default, which breaks backward compatibility of generated code.
This makes it practically infeasible to adopt the latest version of the Typescript generator in my codebase because it contains hundreds of such references.

There have been a number of bug reports on that like https://github.com/OpenAPITools/openapi-generator/issues/19066
and a legacy value for enumPropertyNaming flag has been introduced for the Java generator with https://github.com/OpenAPITools/openapi-generator/pull/20172

The goal of this issue is to port the same flag to the Typescript generator too.

openapi-generator version

7.7.0 - latest (7.12.0)

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/19066
https://github.com/OpenAPITools/openapi-generator/pull/20172

Inspect the TypeScript generator's enum naming handling and compare it with the Java generator's legacy enumPropertyNaming support described in pull request #20172. Use issue #19066 for compatibility context; done means the TypeScript generator offers the requested legacy naming option alongside its current behavior.