[Feature]: Add char support for key events

[bernhardfritz]

Describe your idea

As requested by iddq_tea on discord https://discord.com/channels/959169147550445598/959169147550445601/1041456301311529042, they want an easy way to find which character was last pressed.

Solution:
Add a variable key that contains last ASCII-based key pressed. We might also need another callback like keyTyped that is called whenever a key is supposed to be typed/repeated. GLFW supports this out-of-the-box: https://www.glfw.org/docs/3.3/group__input.html#gab25c4a220fd8f5717718dbc487828996