Rewrite of the Parser to also get the ComplexChildDocument Tests passing

[Stelzi79]

I have the parser rewritten the following way:

• read in all Lines and put them into TamlKeyValuePairs with the level they are intended (number of \t at the start of the line)
• in a second pass I first put all with 0 indentation into TamlDocument.KeyValuePairs
• if the indentation is larger as the current one I read in a parent value and replaced the Value of the KeyValuePair with a TamlArray and added the Values to the array. All subsequent values are addend to that parent.

All Tests are passing now.

I think I have seen some more opportunities for further unit tests. Some important code paths in my opinion are not covered by them.

Also this PR is based on the branch feature_dotnet

This also makes #26 not allowed because this parser doesn't allow inconstant indentation. There is also no unit test and sensible error handling for this case.

I also annotated some Nullables

[csharpfritz]

Fantastic! Thank you for the help with this