Enhance/correct some examples in expressions.md (#645)

* Correct/reformat example

* Correct/reformat example

* minor tweak

* fix errors in previous commit

* make minor tweaks

* add Point and Rectangle support files

I removed the redundant, hard-coded 1x`Point` and 2x`Rectangle` type declarations, replacing them with the following additions to the "additional-files" folder: Point.cs, Rectangle.cs, and RectangleWithConstructorPointAllocation.cs, and using the appropriate `additionalFiles:` in the examples' metadata.

* Create Point.cs

* Create Rectangle.cs

* Create RectangleWithConstructorPointAllocation.cs

* Apply fixes requested by Jon

* Fix nits found by Jon

Author: RexJaeschke