[lang] support ?: key value notation

#! Trying to use the multiline yaml ? : notation for map entries doesn't give the expected results:

#! This works
? ac
: bd

#! But this does not (it gives ee:null not ee:fg)
? ee
: #@ 'f' + 'g'

#! Nor this (it gives null:ax not ax:bd)
? #@ 'a' + 'x'
: #@ 'b' + 'd'

#! This one gives an error if you uncomment it.
#! ? #@ 'y' + 'z'
#! : asdf