"implements" generates to "extends"

[tehwalris]

To reproduce:

• write a class which implements an interface in a text file
• load the file in forest and save it again
• the "extends" keyword is replaced with "implements"

This seems like it will require special handling to fix, because of this type:

token: SyntaxKind.ExtendsKeyword | SyntaxKind.ImplementsKeyword

The problem is that the values of this property are not ts.Node. There are 1-2 more cases like this in other parts of the AST.