More robust solution for unused control characters

This is a better fix for #6682 - all control characters are now set to be ignored in the escape_defaults keymap. I had previously missed a few (e.g., `^]`, `^-`, `^\`) in both the search and default keymaps. This should fully prevent control characters from getting inserted by typing at the REPL (although they can still get inserted via paste).
Merge pull request #6783 from mbauman/more-unused-bindings

More robust solution for unused control characters