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/impl/declaration.carbon

@@ -11,12 +11,11 @@
 impl bool as Interface;
 
 // CHECK:STDOUT: - filename: declaration.carbon
-// CHECK:STDOUT:   parse_tree: [
-// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
-// CHECK:STDOUT:       {kind: 'ImplIntroducer', text: 'impl'},
-// CHECK:STDOUT:         {kind: 'BoolTypeLiteral', text: 'bool'},
-// CHECK:STDOUT:       {kind: 'ImplTypeAs', text: 'as', subtree_size: 2},
-// CHECK:STDOUT:       {kind: 'IdentifierNameExpr', text: 'Interface'},
-// CHECK:STDOUT:     {kind: 'ImplDecl', text: ';', subtree_size: 5},
-// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT:   ]
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─ImplIntroducer 'impl'
+// CHECK:STDOUT:   │ │ ╭─BoolTypeLiteral 'bool'
+// CHECK:STDOUT:   │ ├─ImplTypeAs 'as'
+// CHECK:STDOUT:   │ ├─IdentifierNameExpr 'Interface'
+// CHECK:STDOUT:   ├─ImplDecl ';'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)