TemplateString: Proper inference similar to JoinedStr._infer and additional tests for the as_string()

Fix this error in pylint with 3.14:

-  AttributeError: 'TreeRebuilder' object has no attribute 'visit_templatestr'

https://docs.python.org/3.14/library/ast.html#ast.TemplateStr