App: extend Expression syntax
* Support sub-object reference syntax using the following syntax,
Part.<<Box.>>.Placement
or, with sub-object label referencing
Part.<<$Cube.>>.Placement
* Extend indexing support, including range based indexing, e.g.
A1[B2+1][C3][D4:-1]
* Add new constants, None, True, true, False, false.
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