internal: fix and force-disable block validation ;-(

Originally we tried to maintain the invariant that `{}` always match.
That is, that in the parse tree the pair of corresponding `{}` is always
first and last tokens of some nodes.

We had the code to validate that, but apparently it's been broken for
**years** since we introduced tokens/nodes split. Fixing it now makes
some tests fail.

It's unclear if we want to keep this invariant: there's a strong
motivation for breaking it in the following case:

```
use std::{ // unclosed paren

fn main() {

}

} // don't actually want to pair up this with the one from `use`
```

So let's fix the code, but disable it for the time being

crates/syntax/src/syntax_node.rs

@@ -47,7 +47,7 @@ impl SyntaxTreeBuilder {
 
     pub fn finish(self) -> Parse<SyntaxNode> {
         let (green, errors) = self.finish_raw();
-        if cfg!(debug_assertions) {
+        if cfg!(debug_assertions) && false {
             let node = SyntaxNode::new_root(green.clone());
             crate::validation::validate_block_structure(&node);
         }