Update the static extensions proposal to rely on generic constructors (#4212)

Rename 'feature-specification-variant2.md' to 'feature-specification.md', based on the assumption that this proposal is more likely to have broad support than the other proposal.

This PR rewrites the contents of that feature specification such that it relies on generic constructors #4265, which simplifies the proposal significantly. It can be adjusted to work with other proposals about generic constructors.

The mest perspicuous simplification is the following: If multiple accessible extension declarations with on-declaration `C` declare a constructor named `C.name` (or named `C`), and `C` itself doesn't declare such a constructor then an invocation of `C.name(...)` (respectively `C(...)`), as well as variants with actual type arguments, is an error because of the ambiguity. No attempt is made to determine that some extensions are "more specific" than others.

Author: eernstg