Fix xstr with plain gvar unparsing

Follow-up to #387

```shell
bundle exec bin/unparser -e '`#$G`'
(string)
Original-Source:
`#$G`
Generated-Source:
lib/unparser/emitter/xstr.rb:56:in 'Unparser::Emitter::XStr#escape_xstr'
lib/unparser/emitter/xstr.rb:52:in 'Unparser::Emitter::XStr#emit_string'
lib/unparser/emitter/xstr.rb:45:in 'block in Unparser::Emitter::XStr#emit_xstr'
lib/unparser/emitter/xstr.rb:41:in 'Array#each'
lib/unparser/emitter/xstr.rb:41:in 'Unparser::Emitter::XStr#emit_xstr'
lib/unparser/emitter/xstr.rb:16:in 'Unparser::Emitter::XStr#dispatch'
lib/unparser/generation.rb:13:in 'block in Unparser::Generation#write_to_buffer'
lib/unparser/generation.rb:44:in 'Unparser::Generation#with_comments'
lib/unparser/generation.rb:13:in 'Unparser::Generation#write_to_buffer'
<internal:kernel>:91:in 'Kernel#tap'
lib/unparser/generation.rb:236:in 'Unparser::Generation#visit_deep'
lib/unparser/generation.rb:132:in 'block in Unparser::Generation#emit_body'
lib/unparser/generation.rb:138:in 'Unparser::Generation#with_indent'
lib/unparser/generation.rb:122:in 'Unparser::Generation#emit_body'
lib/unparser/emitter/root.rb:11:in 'Unparser::Emitter::Root#dispatch'
lib/unparser/generation.rb:13:in 'block in Unparser::Generation#write_to_buffer'
lib/unparser/generation.rb:44:in 'Unparser::Generation#with_comments'
lib/unparser/generation.rb:13:in 'Unparser::Generation#write_to_buffer'
lib/unparser.rb:148:in 'block in Unparser.unparse_ast'
<internal:kernel>:91:in 'Kernel#tap'
Original-Node:
(xstr
  (gvar :$G))
Generated-Node:
undefined
Error: (string)
```

Author: viralpraxis

lib/unparser/emitter/xstr.rb

@@ -41,6 +41,8 @@ def emit_xstr
         children.each do |child|
           if n_begin?(child)
             emit_begin(child)
+          elsif n_gvar?(child)
+            emit_gvar(child)
           else
             emit_string(child)
           end
@@ -67,6 +69,11 @@ def emit_begin(component)
         visit(Util.one(component.children)) if component.children.any?
         write('}')
       end
+
+      def emit_gvar(component)
+        write('#')
+        write(Util.one(component.children).to_s)
+      end
     end # XStr
   end # Emitter
 end # Unparser

test/corpus/literal/literal.rb

@@ -74,3 +74,4 @@
 #`
 `#{}`
 `#G`
+`#$G`