Strict Typescript output of discriminator (PHNX-805) (#1)

[gbrown-ce]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Motivation

The current release of swagger-codegen does not output valid strict typescript for discriminators

Modifications

Updating the discriminator output to be valid strict typescript

[gbrown-ce]

@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny

There is a similar PR that has been stagnant for a couple of months (#7824), so I figured I'd try to get this put through.