refactor(resolve): Split implementation into distinct Node types

[gwhitney]

This refactor allows custom resolve methods for different node types
or for newly added node types, as an alternative to #2775.
Adds a test that demonstrates the creating of a (stub) IntervalNode,
as if you were doing interval arithmetic, say, with a custom resolve
method for the new type.

[gwhitney]

P.S. for whomever reviews this: Note that I had to touch typed.js for two reasons:

1. Previously, Node was not at the end of the list of Node types, where it has to be for specific node-type implementations to take precedence over a generic Node implementation, as we want it to.
2. The previously internal Set argument in resolve now has to be exposed in the typed-function interface (but I still didn't document it because it is only used for internal cycle detection). But this happens to be the first time a Set has been used in a mathjs typed-function, so I had to add that type.