Add basic pipeline expressions grammar definition

lib/gitlab/ci/pipeline/expression/lexer.rb

@@ -2,6 +2,14 @@ module Gitlab
   module Ci
     module Pipeline
       module Expression
+        GRAMMAR = [
+          %w[variable equals string],
+          %w[variable equals variable],
+          %w[variable equals null],
+          %w[string equals variable],
+          %w[null equals variable],
+        ]
+
         class Lexer
           def initialize(statement)
             @statement = statement