Add support for some CSI 27 modified variants of Enter

xterm has 3 different disambiguated key modes intended to be used by
applications that wish to distinguish keyboard modifiers for certain
keypresses. These emit CSI 27 escapes with a modifiers bitmask field,
and may be enabled by configuration.

We can't parse these properly, so just add support for a few common
modified variants of Enter in the escape sequence list. These variants
previously had no distinct encoding, so the CSI variants have been
adopted by some other terms, namely foot.