Generate TypeScript 2.4 string enums from Java enums

[vojtechhabarta]

This PR adds possibility to generate TypeScript string enums from Java enums. TypeScript enums were originally number-based but TypeScript 2.4 will allow string members in enums.

Java enum values usually translates to strings in JSON and typescript-generator generates by default union of string literals. For example for this Java enum:

enum Direction {
    Left, Right
}

following TypeScript union type is generated:

type Direction = "Left" | "Right";

This PR adds new possibility to generate following string enum:

export const enum Direction {
    Left = "Left",
    Right = "Right",
}

Configuration

This feature can be used by configuring mapEnum parameter to asEnum value.
For implementation files (.ts) it is also possible to generate enums without const keyword by setting nonConstEnums parameter to true.

Number enums

Some Java enums are rendered as numbers in JSON (for example Jackson2 provides JsonFormat.Shape.NUMBER_INT). Mapping of those enums are not changed by this PR, they are always treated as TypeScript number-based enums.

Deprecated extensions

With this new functionality following extensions are not needed anymore and are deprecated:

• cz.habarta.typescript.generator.ext.EnumConstantsExtension
• cz.habarta.typescript.generator.ext.NonConstEnumsExtension