Make the tree structure more explicit in parse dumps (#7591)

It's hard to track 2-space indent levels across large vertical gaps, and
it can be hard to suppress the instinct to read the dump as if it were
preorder. This change introduces a new dump mode that addresses both
problems by using box-drawing characters to explicitly represent the
parent-child edges of the tree. This new mode is the default, but the
old behavior remains available with
`--parse-dump-format=yaml-postorder`.

toolchain/parse/tree.cpp

@@ -28,7 +28,7 @@ auto Tree::node_token(NodeId n) const -> Lex::TokenIndex {
 }
 
 auto Tree::Print(llvm::raw_ostream& output) const -> void {
-  TreeAndSubtrees(*tokens_, *this).Print(output);
+  TreeAndSubtrees(*tokens_, *this).PrintYamlPostorder(output);
 }
 
 auto Tree::Verify() const -> ErrorOr<Success> {