Add binding for Regexp#fixed_encoding?

It turned out we already had the method in C++, we just did not disclose
it to Ruby space.

lib/natalie/compiler/binding_gen.rb

@@ -1218,6 +1218,7 @@ def generate_name
 gen.binding('Regexp', 'casefold?', 'RegexpObject', 'casefold', argc: 0, pass_env: true, pass_block: false, return_type: :bool)
 gen.binding('Regexp', 'encoding', 'RegexpObject', 'encoding', argc: 0, pass_env: false, pass_block: false, return_type: :Object)
 gen.binding('Regexp', 'eql?', 'RegexpObject', 'eq', argc: 1, pass_env: true, pass_block: false, aliases: ['=='], return_type: :bool)
+gen.binding('Regexp', 'fixed_encoding?' ,'RegexpObject', 'is_fixed_encoding', argc: 0, pass_env: false, pass_block: false, return_type: :bool)
 gen.binding('Regexp', 'hash', 'RegexpObject', 'hash', argc: 0, pass_env: true, pass_block: false, return_type: :Object)
 gen.binding('Regexp', 'initialize', 'RegexpObject', 'initialize', argc: 1..2, pass_env: true, pass_block: false, return_type: :Object, visibility: :private)
 gen.binding('Regexp', 'inspect', 'RegexpObject', 'inspect', argc: 0, pass_env: true, pass_block: false, return_type: :Object)