Ruby: Added missing keywords (#2097)

This adds the missing keywords `extend`, `include`, and `prepend`.

components/prism-ruby.js

@@ -13,7 +13,7 @@
 				greedy: true
 			}
 		],
-		'keyword': /\b(?:alias|and|BEGIN|begin|break|case|class|def|define_method|defined|do|each|else|elsif|END|end|ensure|false|for|if|in|module|new|next|nil|not|or|protected|private|public|raise|redo|require|rescue|retry|return|self|super|then|throw|true|undef|unless|until|when|while|yield)\b/
+		'keyword': /\b(?:alias|and|BEGIN|begin|break|case|class|def|define_method|defined|do|each|else|elsif|END|end|ensure|extend|false|for|if|in|include|module|new|next|nil|not|or|prepend|protected|private|public|raise|redo|require|rescue|retry|return|self|super|then|throw|true|undef|unless|until|when|while|yield)\b/
 	});
 
 	var interpolation = {

tests/languages/ruby/keyword_feature.test

@@ -15,16 +15,19 @@ elsif
 END
 end
 ensure
+extend
 false
 for
 if
 in
+include
 module
 new;
 next
 nil
 not
 or
+prepend
 protected
 private
 public
@@ -66,16 +69,19 @@ yield
 	["keyword", "END"],
 	["keyword", "end"],
 	["keyword", "ensure"],
+	["keyword", "extend"],
 	["keyword", "false"],
 	["keyword", "for"],
 	["keyword", "if"],
 	["keyword", "in"],
+	["keyword", "include"],
 	["keyword", "module"],
 	["keyword", "new"], ["punctuation", ";"],
 	["keyword", "next"],
 	["keyword", "nil"],
 	["keyword", "not"],
 	["keyword", "or"],
+	["keyword", "prepend"],
 	["keyword", "protected"],
 	["keyword", "private"],
 	["keyword", "public"],