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/generic_params/basic.carbon

@@ -11,16 +11,15 @@
 fn foo(generic a: i32);
 
 // CHECK:STDOUT: - filename: basic.carbon
-// CHECK:STDOUT:   parse_tree: [
-// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
-// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:       {kind: 'IdentifierNameMaybeBeforeSignature', text: 'foo'},
-// CHECK:STDOUT:         {kind: 'ExplicitParamListStart', text: '('},
-// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeSignature', text: 'a'},
-// CHECK:STDOUT:           {kind: 'CompileTimeBindingPatternTypeStart', text: ':'},
-// CHECK:STDOUT:           {kind: 'IntTypeLiteral', text: 'i32'},
-// CHECK:STDOUT:         {kind: 'CompileTimeBindingPattern', text: ':', subtree_size: 4},
-// CHECK:STDOUT:       {kind: 'ExplicitParamList', text: ')', subtree_size: 6},
-// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 9},
-// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
-// CHECK:STDOUT:   ]
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─FunctionIntroducer 'fn'
+// CHECK:STDOUT:   │ ├─IdentifierNameMaybeBeforeSignature 'foo'
+// CHECK:STDOUT:   │ │ ╭─ExplicitParamListStart '('
+// CHECK:STDOUT:   │ │ │ ╭─IdentifierNameNotBeforeSignature 'a'
+// CHECK:STDOUT:   │ │ │ ├─CompileTimeBindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │ │ ├─IntTypeLiteral 'i32'
+// CHECK:STDOUT:   │ │ ├─CompileTimeBindingPattern ':'
+// CHECK:STDOUT:   │ ├─ExplicitParamList ')'
+// CHECK:STDOUT:   ├─FunctionDecl ';'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)