Convert usages of `get Node.disposed` to use `Node.isDisposed`

[samreid]

From phetsims/scenery#899, we would like to be consistent about using isDisposed instead of disposed. Some cases in Build an Atom declare their own subtype attribute for disposed, even though they extend a type that already tracks isDisposed such as Node or PhetioObject. See also comment in phetsims/scenery#899 (comment)

Assigning to responsible dev for investigation.

[jbphet]

Investigated, changed usages of dispose to isDisposed. I don't think this requires a review, but @samreid is welcome to take a look if he'd like. Closing.