We could similarly add others -- this is intended to make it easy to add more that parse essentially the same.
The functionality expected is that types will use GetDeclarationContext in order to error on certain functionality in the declaration scope loop. e.g., with how constraints and interfaces currently don't allow definitions.
I've only moved out `package` because it's only valid on the top line. It might still be good to parse it later, but with slightly different logic because it would always be an error, and the declaration context isn't quite the right framing for that.
Also unifies some errors with `fn`.
The comments in parse_node_kind.def capture the change being made here.
Before:
```
// _external_: DeclaredName
// InterfaceBodyStart
// _external_: statements
// InterfaceBodyEnd
// InterfaceDefinition
```
After:
```
// InterfaceIntroducer
// DeclaredName
// InterfaceDefinitionStart
// _external_: declarations
// InterfaceDefinition
```
Really I just want to treat introduced things consistently. `var` defines my philosophy here: it doesn't always have a `DeclaredName`, so the `VarIntroducer` _must_ be the bounding node. By being consistent with that, I believe that overall the structure becomes easier to understand (that is, there are fewer inconsistencies to understand).
This also adds InterfaceDeclaration, since I think it can be predicted we'll have that, and it's helpful for making recover consistent with HandleDeclarationError.
Similarly, I'm also trying to standardize the loop processing a little with HandleDeclarationLoop. In the current approach, InterfaceDefinitionFinish isn't a necessary state, so I'm removing it.
First change towards re-implementing interfaces using the new parser. I kept it small to make sure we are on the same page regarding stack states and how the parse tree should look like.
Co-authored-by: ergawy <kareem.ergawy@guardsquare.com>
Jon Ross-Perkins
Kareem Ergawyandergawy