Adding a default font

[felixmariotto]

So far manually adding a font for three-mesh-ui to create text is mandatory.

It must be defined this way :

ThreeMeshUI.Block({
 fontFamily: './assets/Roboto-msdf.json',
 fontTexture: './assets/Roboto-msdf.png'
});

It is not very user-friendly, as the user must download the fonts from the repo or take the time to create their own, find a place to store it in their app file system, etc...

I think it would be best to have at least the Roboto font (open source) as the default font, so these two parameters can be skipped, and Text will fallback to Roboto if the user defined no font.

The font files should be placed in ./src/assets