Add shift keymap

[PointerDilemma]

Using underscores in test names is pretty common.

Now that sweb has tab completion, you can select tests with underscores in their name without having to type them. Which lead to groups being confused why the tortillas test runner cannot run their tests.

So I thought it would be good to make underscores typeable by default.

Instead of remapping the keys with Terminal::remap, this introduces a shift map based on the US layout. So other characters like :"|<>?+ are also typeable.

If this is not desirable, I would propose to just add underscores to the remap function like this.

Idk about the rpi2 and rpi3 implementations, their E0 map is weird, so I did not even touch that. This will break shift for them.