[PRISM] Implement `PM_IMAGINARY_NODE` for `defined?`

Ruby Code:

```
defined?(1i)
```

Instructions:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:0 (0,0)-(0,12)>
0000 putobject                              "expression"
0002 leave

"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:0 (0,0)-(0,12)>
0000 putobject                              "expression"
0002 leave
```

prism_compile.c

@@ -1483,6 +1483,7 @@ pm_compile_defined_expr0(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *co
       case PM_AND_NODE:
       case PM_FLOAT_NODE:
       case PM_HASH_NODE:
+      case PM_IMAGINARY_NODE:
       case PM_INTEGER_NODE:
       case PM_INTERPOLATED_REGULAR_EXPRESSION_NODE:
       case PM_INTERPOLATED_STRING_NODE:

test/ruby/test_compile_prism.rb

@@ -96,6 +96,7 @@ module Prism
       assert_prism_eval("defined? true")
       assert_prism_eval("defined? false")
       assert_prism_eval("defined? 1")
+      assert_prism_eval("defined? 1i")
       assert_prism_eval("defined? 1.0")
       assert_prism_eval("defined? 1..2")
       assert_prism_eval("defined? [A, B, C]")