support separating DocString with ```
[csharp] add new ast classes
[csharp] fix test setup
[csharp] improve test formatter
[csharp] build Step (w/o args)
[csharp] improve testing and formatter
[csharp] build Scenario (w/o Tags and description)
update expected output
[csharp] handle EOF -> skip
[csharp] build feature header, description
[csharp] add parsertester command line tool
move testdata to the right place, rename result to ast
[csharp] rename tester too to AstTester