Add helper method to coerce attribute values

[mojavelinux]

Add a helper method to coerce attribute values so that they are assigned in JavaScript as the appropriate type and safely.

There are at least two cases, currently.

1. the attribute value needs to be emitted as a quoted string
2. the attribute value needs to be emitted as a boolean

In the second case, the backend is currently broken if the attribute value is not assigned a value of "true" or "false". For example, consider the following attribute declaration:

:revealjs_history:

This produces:

history: ,