[Typescript] Enum types (#18531)

[wing328]

based on #18531 (originally targeting 8.0.x branch)

---

Summary by cubic

Adds a new enumType option to the TypeScript client generator to choose between TypeScript enums and string literal unions. Default remains enum for compatibility; consolidated TypeScript configs now use stringUnion.

• New Features

  • New CLI/config option enumType with values enum (default) or stringUnion.
  • Templates updated to emit export enum or export type Foo = "a" | "b".
  • Docs updated; consolidated TS sample configs set enumType: stringUnion; tests adjusted, including a case with no option set to verify the default.

• Migration

  • No changes needed if you rely on export enum (default).
  • To use union types, set additionalProperties.enumType=stringUnion and update usages from Enum.Member to string literals (e.g., 'available').

^{Written for commit 2307594. Summary will update on new commits.}