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

[vladi-velikov]

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 #19066
and a legacy value for enumPropertyNaming flag has been introduced for the Java generator with #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

#19066
#20172

[vladi-velikov]

@wing328 Could you please check if it is possible to port the legacy flag to Typescript generator?
Thanks!

[gcatanese]

Does PascalCase work for you? If not, how does legacy work? What do you expect?