Add an Interface SpaceKind

[dburriss]

I am opening this issue to have a discussion and record my findings. This may be something we don't want to do.

The idea is to add SpaceKind::Interface enum value so that interfaces show like classes (see image below).

This was kicked off from the conversation here with @calixteman .

Scope and motivation
This would be used in supported languages that make use interfaces ie. Java, TypeScript (future C# and Go?). This makes more sense recently since, for good or ill, many languages support default implementations on interfaces. This would give other languages a little more parity with the Rust implementation which distinguishes trait and impl already.

Effort and impact
This appears to be a fairly trivial feature to add:

• Add SpaceKind::Interface enum
• Add SpaceKind::Interface => "interface", to the match in the SpaceKind Display trait's fmt implementation

The following tasks are needed per language

• For language's Getter.get_space_kind return SpaceKind::Interface for correct grammar elements ie. InterfaceDeclaration in Java ie. InterfaceDeclaration => SpaceKind::Interface
• For the language's Checker implementation, add the grammar elements in the previous task to the is_func_space mk_checker macro arguments so the interface is included as a space for functions and metrics

I am not sure if this has much of a performance impact? I imagine not because of the type of change but I would need input or time to run tests.

Result
I implemented this for the Java lang and this is what it looks like.

[dburriss]

Some open questions for me on this:

• Do we even want to do this? Support this?
• Perf impact?
• Is this a breaking change? It does change the structure of output so are people depending on metrics being in only Unit, Class, or Function?
• How can this be tested?

[dburriss]

Closed by #810