Fix JS prototype chain for exported .NET subclasses

[jasongin]

Fixes: #365

• In the source generator, pass the base class to JSClassBuilder constructor baseClass parameter when defining a subclass.
  • This sets up the JS prototype chain correctly, enabling access to base members on the subclass and making JS instanceof work as expected for instances of the exported classes.
  • This required some significant refactoring in ModuleGenerator.cs to ensure the base class was separately declared as a variable before constructing the subclass. Previously the entire module was declared as one chained statement; now each type is declared and added in separate statements.
• In the source generator, include base interface members in the generated interface proxy classes.
• Change how the version.props file is generated by dotnet pack.
  • This isn't directly related to the above fixes, but it has been annoying me during development because the previous mechanism sometimes did not regenerate the version after switching git branches.