Exclude commutatively merging of comments by default #467

Follow-up to #459. At the moment, all commutative parent declarations ought to be restricted to the children types they are meant to contain, so that commutative merging of comments is excluded. This is because the positioning of comments is something that carries meaning and the AST structure doesn't encode that meaning. But it's annoying to have to list explicitly all possible children types of a node. If a new feature is added in the grammar later on, we will probably not notice it and forget to add it to the list. An 'extra' node is a node type declared in the definition of the tree-sitter grammar, that can appear anywhere: https://tree-sitter.github.io/tree-sitter/creating-parsers/2-the-grammar-dsl.html A solution would be to refuse commutatively merging any 'extra' node, but at the moment it's not possible to detect if a node is an 'extra' or not. But I am thinking of another solution: it looks like all possible children of a given node type can be looked up programmatically from the `NODE_TYPES` constant exposed by grammars (which does not include the extras). We could restrict commutativity to those nodes listed there. It's non-trivial though, because it requires expanding the declared set of possible children via the subtypes. I've opened a feature request in tree-sitter to expose the `extras` list through bindings: https://github.com/tree-sitter/tree-sitter/issues/4556