fix: 区分字符和字符串 (#74)

Co-authored-by: Huaps <1183155719@qq.com>
Reviewed-on: PostGuard/Canon#74

Canon.Tests/GrammarParserTests/PascalGrammarTests.cs

@@ -39,7 +39,7 @@ public class PascalGrammarTests
     {
         const string program = """
                                program exFunction;
-                               const str = 'result is : ';
+                               const str = 'a';
                                var a, b : Integer;
                                begin
                                writeln( str, ret );