[typescript] Introduce "enumPropertyNaming" option

[amakhrov]

This works similar to the existing "modelPropertyNaming", but applies to enum var names instead of model properties.
Default value is "PascalCase", which reflects the way it works currently.

Personally, I only need this configurable in the typescript-angular generator now. However, since the enum var name transformation is implemented in the abstract typescript class, I thought it would be beneficial to keep consistency across multiple typescript generators. After all, this is quite a generic setting, not specific to a particular framework.

PR checklist

• Read the contribution guidelines.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
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@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @nicokoenig @topce @akehir @petejohansonxo

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[macjohnny]

note: instead of fake commits, you can also close and re-open the PR to trigger CI

[macjohnny]

maybe the available values in ENUM_PROPERTY_NAMING_TYPE could be listed automatically, so the comment will not be outdated

[amakhrov]

Good idea, will do!

[amakhrov]

and done

[macjohnny]

maybe you would want to add a test for the non-original case, too?

[amakhrov]

Done

[macjohnny]

there seems to be some issue with the option parsing, see https://app.shippable.com/github/OpenAPITools/openapi-generator/runs/14327/1/console

[amakhrov]

there seems to be some issue with the option parsing

Looks like Kotlin build is broken - no idea why. Rebasing my branch from the latest master to see if it's gonna self-resolve

[amakhrov]

The last CI step still fails with a kotlin-related error.
I can see that some other open PRs also fail with the same error. Is it a global issue with configuration / 3rd party service?