Add type declarations

[vitonsky]

Hey Jon. This PR adds a type declarations in build, to make this code works in typescript code.

Additionally typescript compiler found a mismatch segment implementation to a base class type, since it must return object with method containing and iterator symbol. I've fixed that problem with stub implementation of containing that throws error if will be used. We may create issue to implement this method later.

Now code compiles well. Example of generated .d.ts file is

declare class Segmenter extends Intl.Segmenter {
    private readonly language;
    private readonly options;
    constructor(language: string, options?: Intl.SegmenterOptions & {
        maxChunkLength?: number;
    });
    segment(input: string): Intl.Segments;
    findSafeBreakPoint(input: string): number;
    getSegments(input: string): Intl.SegmentData[];
    static getSegments(input: string, language: string, options?: Intl.SegmenterOptions): Intl.SegmentData[];
}

export { Segmenter };

Please update package once this changes will be merged. I faced with a performance problem that solves your package, so I would be happy to migrate on your solution.