Allow trailing dot on arbitrary expressions

[bentsherman]

Enables auto-completion on more kinds of expressions, for example:

Channel.of(1, 2, 3). // map, filter, collect...

The challenge is that expression also allows incompleteExpression, but the inner expression of incompleteExpression should not allow further incomplete expressions.

The current changes to the script parser feel messy and also are incomplete. It would probably be better to parametrize the expression rule with a flag for whether the incompleteExpression is allowed.