Adding Super and Hyper sequences to modifiers

As requested on: https://www.reddit.com/r/BlinkShell/comments/17zhk8d/emacs_keys_setup/

Super and Hyper can be supported through custom key sequences. The problem is this would require a lot of sequences to be defined for this scenario. It would be interesting to have a "Custom" option in the keyboard configuration to prepend characters to a modifier.

References: https://www.gnu.org/software/emacs/manual/html_node/emacs/Modifier-Keys.html

Start with Blink’s keyboard configuration and modifier handling, focusing on how custom key sequences are currently defined. Compare the desired Super and Hyper behavior with the GNU Emacs modifier reference; the work is done when a Custom option can prepend characters to a modifier without requiring every sequence to be defined separately.

Written by the indexing model from the issue text.