Move grammar parsing into new Grammar class

lib/propane/token.rb

@@ -14,14 +14,29 @@ class Propane
     #   Token ID.
     attr_reader :id
 
+    # @return [Integer, nil]
+    #   Line number where the token was defined in the input grammar.
+    attr_reader :line_number
+
     # @return [Regex::NFA]
     #   Regex NFA for matching the token.
     attr_reader :nfa
 
-    def initialize(name, pattern, id)
+    # Construct a Token.
+    #
+    # @param name [String]
+    #   Token name.
+    # @param pattern [String]
+    #   Token pattern.
+    # @param id [Integer]
+    #   Token ID.
+    # @param line_number [Integer, nil]
+    #   Line number where the token was defined in the input grammar.
+    def initialize(name, pattern, id, line_number)
       @name = name
       @pattern = pattern
       @id = id
+      @line_number = line_number
       unless pattern.nil?
         regex = Regex.new(pattern)
         regex.nfa.end_state.accepts = self