Interop Gen: Add Support for Merging Interface Types to already built-in Dart JS Interop Types as Extensions

[nikeokoronkwo]

Some interfaces established in dart:js_interop may be extended by a library via an interface, such as how the ES specs extend each other, and already built in interfaces that dart:js_interop may have.

// from: https://github.com/microsoft/TypeScript/blob/main/src/lib/es2015.core.d.ts
interface Array<T> {
    find<S extends T>(predicate: (value: T, index: number, obj: T[]) => value is S, thisArg?: any): S | undefined;
    find(predicate: (value: T, index: number, obj: T[]) => unknown, thisArg?: any): T | undefined;

    findIndex(predicate: (value: T, index: number, obj: T[]) => unknown, thisArg?: any): number;

    fill(value: T, start?: number, end?: number): this;
    copyWithin(target: number, start: number, end?: number): this;

    toLocaleString(locales: string | string[], options?: Intl.NumberFormatOptions & Intl.DateTimeFormatOptions): string;
}

Since Dart's JS Interop has support for external extensions to such types, we can convert these into extensions on types in dart:js_interop types during overload merging (#388)