Add VirtualKeyboard support to InputTextArea

[ycaptain]

This PR adds VirtualKeyboard support to InputTextArea.

I check if this._currentlyConnectedInputText instanceof InputTextArea to call matching method this._currentlyConnectedInputText.alternativeProcessKey, or call processKey for InputText.

Besides, pass different parameters for the input of Enter and Backspace.

inputText.processKey(8);
inputTextArea.alternativeProcessKey('Backspace');

So that an InputTextArea could be connected by a VirtualKeyboard and respond to virtual keyboard input.

const textarea = new InputTextArea('Textarea');
const keyboard = new VirtualKeyboard('Keyboard');
keyboard.connect(textarea);

Discussed in https://forum.babylonjs.com/t/improve-gui-virtualkeyboard-to-support-inputtextarea/32108.

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Snapshot stored with reference name:
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[deltakosh]

Thanks!!