Allow unknown function calls to specify an expression type

[christopherswenson]

Adds new syntax:

• function_name!number!measure(...) to mark that the function should be treated as an aggregate expression, including handling distinct keys for joins
• join1.join2.function_name!number!measure(...)
• join1.join2.field1.function_name!number!measure(...)
• field1.function_name!number!measure(...)
• function_name!number!calculation(...) to mark that the function should be treated as an analytic function, including adding an OVER (...) clause afterward, and support for { partition_by: ..., order_by: ... }
• function_name!number!dimension(...) is the same as function_name!number(...) but more explicit

This all requires the ##! experimental.unknown_function_expression_types experiment to be enabled.

[christopherswenson]

Not sure about measure/calculation vs aggregate/analytic...