Improve typings for methods that return a Delta

[younggglcy]

my current quill version: v2.0.0.

as in v2, quill provides its own bundled dts file for developers, but some methods under the Quill scope are incorrectly typed due to Function.call() will returns an any type.

e.g., quill.updateContents(), its return type should be Delta, but in quill.d.ts, the signature is

declare class Quill {
  updateContents(delta: Delta | Op[], source?: EmitterSource): any;
}

this pr solves the problem by explicitly declaring the return type, and improve dts type tests

[luin]

Thanks for the contribution!