main/postmarketos-cros-common: add keyboard mapping

Adds custom keyboard mapping documented on https://wiki.postmarketos.org/wiki/Category:ChromeOS#Keyboard_mapping

Testing is appreciated. Can be tested with mrtest installing all the packages it suggests. Once the MR is merged you should remove it with mrtest zap and then install properly with apk add postmarketos-cros-keymap to keep your installation clean