Add AST display as a "fake target"

[sschuberth]

Sometimes it's useful to see the Abstract Syntax Tree (AST) for some code, e.g. when writing detekt / ktlint rules. It would be great if the playground could add the ASP display as a "fake target" and make it look similar to ktlint's (by now removed again, in favor of PsiViewer) --print-ast output.