[HTML5] Add easy to use download API.

[Faless]

New JavaScript.download_buffer method to create a prompt that let the user download a file.

extends Node

func _ready():
	JavaScript.download_buffer("My File Content".to_utf8(), "file.txt")

This PR depends on #48719, since too many renames happened there.
This branch won't build unless #48928 is cherry-picked into it.

There is a 3.x version here: /~https://github.com/Faless/godot/tree/js/3.x_download_buffer