Interfaces syntax

[Kronos3]

Implements the syntax for interfaces.

Generalized the SpecTopImport AST Node type into a SpecImport so that we could reuse the same grammar rule in both types of imports. The nodes themselves are still different in the final AST so that visitors don't need additional context.

Closes #698

[bocchino]

Looks good! Before we merge this, can you please complete the process listed here:

https://github.com/nasa/fpp/wiki/Adding-New-Features-to-FPP#implementing-and-testing-new-syntax

Specifically:

1. Update the AST writer
2. Update the FPP writer
3. Add unit tests for fpp-syntax
4. Add unit tests for fpp-format

[bocchino]

Looks good!