Implement model default value

[Implement values grammar · Issue #2 · happenslol/tree-sitter-typespec](https://github.com/happenslol/tree-sitter-typespec/issues/2) I thought it was the same as the issue above, but this was a different story. [Models | TypeSpec](https://typespec.io/docs/language-basics/models/) ``` tsp model Dog { address?: string = "wild"; } ```