Added prototype of InputTextArea

[fneitzel]

As per request I send this proposal of an InputTextArea. So one can have a multiple line InputTextField. Here is a link to the forum discussion: https://forum.babylonjs.com/t/gui-textinput-with-multiple-lines/9927

The are a quite a few things left and since I got no time in the moment, maybe others can push this forward.

What's missing:

include margin
text highlighting (copy/paste/delete)
backspace deletion

I don't know how to reolve this issue:

 [at-loader] ../../gui/src/2D/controls/inputTextArea.ts:906:55 
    TS2339: Property 'text' does not exist on type 'object'.

[deltakosh]

Hello and thanks a ton! Before going in depth I think we should consider having the textarea inheriting from InputText as they share a lot of code (to avoid duplications and maintenance hurdles)

[sebavan]

Related to #5630

[sebavan]

It is getting there nicely, text highlighting and the rest would be kickass :-)

[sebavan]

@fneitzel do you think you will address the PR ?

[deltakosh]

Closing for being stalled for too long

Please feel free to reopen when possible