Move pipeline expressions grammar from lexer to parser

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

@@ -2,14 +2,6 @@ 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

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

@@ -3,6 +3,14 @@ module Gitlab
     module Pipeline
       module Expression
         class Statement
+          GRAMMAR = [
+            %w[variable equals string],
+            %w[variable equals variable],
+            %w[variable equals null],
+            %w[string equals variable],
+            %w[null equals variable],
+          ]
+
           def initialize(pipeline, statement)
             @pipeline = pipeline
             @statement = statement