[Prism] Implement defined? for PM_BEGIN_NODE

Ruby code:

```ruby
defined?(begin; 1; end)
```

Instructions:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(59,23)>
0000 putobject                              "expression"              (  59)[Li]
0002 leave
"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:58 (58,0)-(58,23)>
0000 putobject                              "expression"              (  58)[Li]
0002 leave
```

Related: ruby/prism#2188

prism_compile.c

@@ -2462,6 +2462,7 @@ pm_compile_defined_expr0(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *co
           }
       }
       case PM_AND_NODE:
+      case PM_BEGIN_NODE:
       case PM_BREAK_NODE:
       case PM_DEFINED_NODE:
       case PM_FLOAT_NODE:

test/ruby/test_compile_prism.rb

@@ -225,6 +225,8 @@ module Prism
       assert_prism_eval("defined?(return)")
       assert_prism_eval("defined?(retry)")
 
+      assert_prism_eval("defined?(begin; 1; end)")
+
       assert_prism_eval("defined?(defined?(a))")
       assert_prism_eval('defined?(:"#{1}")')
       assert_prism_eval("defined?(`echo #{1}`)")