[Generator] Rendering string constants and anyOf with complex members

[nhtruong]

This PR is to address the issues in #886.

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1. Added ability to render string constants:

To add more info for each member of the ExpandWildcard enum, we have converted its schema into an oneOf object where each member is a string constant. This changes causes the generator to create:
type ExpandWildcard = 'string' | 'string' | 'string' | 'string' | 'string'
instead of
type ExpandWildcard = 'all' | 'closed' | 'hidden' | 'none' | 'open'
we're adding another guard to handle const for the renderer.

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2. Added ability to render anyOf objects where members can be anonymous objects:

The InlineScript is now an anyOf object where a member is a string, and another is a complex anonymous object, previously the generator will do:

export type InlineScript = string | extends ScriptBase {
  lang?: ScriptLanguage;
  options?: Record<string, string>;
  source: string;
})

which is not a valid type definition. The changes in render_anyOf and render_allOf functions will render

export type InlineScript = string | (ScriptBase & {
  lang?: ScriptLanguage;
  options?: Record<string, string>;
  source: string;
})

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3. All type definitions are now type instead of a mix of type and interface

Previously the generator would generate interface X whenever possible and would generate type X = when not. The reason? Precedence. So we ended up with a mix of type and interface declarations. These changes [1], [2], [3], simplifies the rendering logic to always render type, which is also a lot more flexible (which enables change No.2 above) and better mirror JSON Schema's allOf

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