Helper to check whether an ast node points to a connection

[rachel-fenichel]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide

The details

Resolves

Proposed Changes

Store whether an ast node points to a connection on the node, and add a getter.

This is a pretty common check and easy to know from the type.

Reason for Changes

Helper function

Test Coverage

Added checks to the ast node constructor tests.

[RoboErikG]

Is this necessary if we're getting rid of insertionNode_ or is it still useful for the tests?

[RoboErikG]

And if it's still helpful then lgtm.