Syntax, type-checking, and runtime support for optional list elements

[TristonianJones]

As a refinement to optional values, this change introduces support for optional list elements during list literal construction:

// If 'b' is present on 'a', then the output will be [a.b, 'hello', 'world']
// else ['hello', 'world']
[?a.?b, 'hello', 'world'] 

There are some minor changes to the grammar to split up the .? and [? tokens so that ? appears as it's own token as otherwise the parser gets confused by a[?b] versus [?a[?b]]

[TristonianJones]

/gcbrun