[PRISM] Implement `PM_SOURCE_ENCODING_NODE` for `defined?

Ruby code:

```ruby
defined?(__ENCODING__)
```

Instructions:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:0 (0,0)-(0,22)>
0000 putobject                              "expression"
0002 leave

"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:0 (0,0)-(0,22)>
0000 putobject                              "expression"
0002 leave
```

prism_compile.c

@@ -1491,6 +1491,7 @@ pm_compile_defined_expr0(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *co
       case PM_OR_NODE:
       case PM_RANGE_NODE:
       case PM_REGULAR_EXPRESSION_NODE:
+      case PM_SOURCE_ENCODING_NODE:
       case PM_STRING_NODE:
       case PM_SYMBOL_NODE:
       case PM_X_STRING_NODE:

test/ruby/test_compile_prism.rb

@@ -110,6 +110,7 @@ module Prism
       assert_prism_eval("defined? -> { 1 + 1 }")
       assert_prism_eval("defined? a && b")
       assert_prism_eval("defined? a || b")
+      assert_prism_eval("defined? __ENCODING__")
 
       assert_prism_eval("defined? %[1,2,3]")
       assert_prism_eval("defined? %q[1,2,3]")