Disallow number and dash identifier prefixes

test/unit/lexer_unit_test.rb

@@ -32,7 +32,10 @@ class LexerUnitTest < Minitest::Test
 
   def test_fancy_identifiers
     tokens = Lexer.new('hi five?').tokenize
-    assert_equal [[:id,'hi'], [:id, 'five?'], [:end_of_string]], tokens
+    assert_equal [[:id, 'hi'], [:id, 'five?'], [:end_of_string]], tokens
+
+    tokens = Lexer.new('2foo').tokenize
+    assert_equal [[:number, '2'], [:id, 'foo'], [:end_of_string]], tokens
   end
 
   def test_whitespace

test/unit/variable_unit_test.rb

@@ -108,6 +108,8 @@ class VariableUnitTest < Minitest::Test
 
     with_error_mode :strict do
       assert_raises(Liquid::SyntaxError) { Variable.new('foo - bar') }
+      assert_raises(Liquid::SyntaxError) { Variable.new('-foo') }
+      assert_raises(Liquid::SyntaxError) { Variable.new('2foo') }
     end
   end