Interactive shell doesn't accept multi-line subshells/brace groups

[emersion]

$ (
build/mrsh:1:1: syntax error: expected a compound list
$ ( echo a
build/mrsh:1:1: syntax error: expected ')'
$ {
build/mrsh:1:1: syntax error: expected a compound list

All of these should allow the user to write contiuation lines.

[emersion]

$ while true
build/mrsh:1:1: syntax error: expected 'do'
$ if true
build/mrsh:1:1: syntax error: expected 'then'

We should detect all "expected XXX, got EOF" cases.