fix TypeScript typings for TextInput methods

[Nkzn]

Motivation

• paper version: v2.6.1
• typescript version: v3.2.2

The TextInput class has public methods isFocused, clear, focus, blur.

https://github.com/callstack/react-native-paper/blob/v2.6.1/src/components/TextInput.js#L348-L374

However, they are not declared in d.ts as below.

Test plan

Typecheck for below function.

import { TextInput } from "react-native-paper";

function doAnything(textInput: TextInput) {
  textInput.isFocused();
  textInput.clear();
  textInput.focus();
  textInput.blur();
}

[satya164]

Thank you for the PR!

[callstack-bot]

Hey @Nkzn, thank you for your pull request 🤗. The documentation from this branch can be viewed here. Please remember to update Typescript types if you changed API.