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/testdata/generics/interface/declaration.carbon

@@ -11,10 +11,9 @@
 interface Foo;
 
 // CHECK:STDOUT: - filename: declaration.carbon
-// CHECK:STDOUT:   parse_tree: [
-// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
-// CHECK:STDOUT:       {kind: 'InterfaceIntroducer', text: 'interface'},
-// CHECK:STDOUT:       {kind: 'IdentifierNameNotBeforeSignature', text: 'Foo'},
-// CHECK:STDOUT:     {kind: 'InterfaceDecl', text: ';', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT:   ]
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─InterfaceIntroducer 'interface'
+// CHECK:STDOUT:   │ ├─IdentifierNameNotBeforeSignature 'Foo'
+// CHECK:STDOUT:   ├─InterfaceDecl ';'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)