[Prism] Implement defined? for PM_SINGLETON_CLASS_NODE

Ruby code:

```ruby
defined?(class << self; end)
```

Instructions:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(59,16)>
0000 putobject                              "expression"              (  59)[Li]
0002 leave
"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:58 (58,0)-(58,16)>
0000 putobject                              "expression"              (  58)[Li]
0002 leave
```

Related: ruby/prism#2188

prism_compile.c

@@ -2565,6 +2565,7 @@ pm_compile_defined_expr0(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *co
       case PM_REGULAR_EXPRESSION_NODE:
       case PM_RETRY_NODE:
       case PM_RETURN_NODE:
+      case PM_SINGLETON_CLASS_NODE:
       case PM_SOURCE_ENCODING_NODE:
       case PM_SOURCE_FILE_NODE:
       case PM_SOURCE_LINE_NODE:

test/ruby/test_compile_prism.rb

@@ -259,6 +259,7 @@ module Prism
       assert_prism_eval("defined?(1 => 1)")
       assert_prism_eval("defined?(module M; end)")
       assert_prism_eval("defined?(1.2r)")
+      assert_prism_eval("defined?(class << self; end)")
     end
 
     def test_GlobalVariableReadNode