use StringScanner to improve Expression Parsing and Tokenizer

test/unit/tokenizer_unit_test.rb

@@ -6,6 +6,7 @@ class TokenizerTest < Minitest::Test
   def test_tokenize_strings
     assert_equal([' '], tokenize(' '))
     assert_equal(['hello world'], tokenize('hello world'))
+    assert_equal(['{}'], tokenize('{}'))
   end
 
   def test_tokenize_variables
@@ -34,6 +35,19 @@ class TokenizerTest < Minitest::Test
     assert_equal([], tokenize(nil))
   end
 
+  def test_incomplete_curly_braces
+    assert_equal(["{{.}", " "], tokenize('{{.} '))
+    assert_equal(["{{}", "%}"], tokenize('{{}%}'))
+    assert_equal(["{{}}", "}"], tokenize('{{}}}'))
+  end
+
+  def test_unmatching_start_and_end
+    assert_equal(["{{%}"], tokenize('{{%}'))
+    assert_equal(["{{%%%}}"], tokenize('{{%%%}}'))
+    assert_equal(["{%", "}}"], tokenize('{%}}'))
+    assert_equal(["{%%}", "}"], tokenize('{%%}}'))
+  end
+
   private
 
   def new_tokenizer(source, parse_context: Liquid::ParseContext.new, start_line_number: nil)