Improve key->string conversion

The strings shown for commands like Get-PSReadlineKeyHandler or
shown from functions WhatIsKey and ShowKeyBindings should now
match what you can use with Set-PSReadlineKeyHandler.

These strings will also omit +Shift in most cases because it
is usually clear from the character, e.g. the following are
equivalent but the former is preferred.

    Ctrl+?
    Ctrl+Shift+/