Add detection of GrammarKit-generated files

GrammarKit is a plugin by JetBrains for creating custom language plugins
for JetBrains IDEs (such as IntelliJ, RubyMine, CLion and more). It
defines a BNF parser language which can be used to generate a parser in
Java, and it also integrates JFLex for generating a lexer in Java.

Both of these generated Java files can be recognised by a comment on the
first line of the file, and so classifying them as generated is trivial.

test/test_generated.rb

@@ -69,5 +69,11 @@ class TestGenerated < Minitest::Test
 
     # Specflow
     generated_fixture_without_loading_data("Features/BindingCulture.feature.cs")
+
+    # JFlex
+    generated_sample_loading_data("Java/JFlexLexer.java")
+
+    # GrammarKit
+    generated_sample_loading_data("Java/GrammarKit.java")
   end
 end