so as not to cause confusion with ASTNode. Also additionally rename
PyLiteralStringElement to PyPlainStringElement, again, not to confuse
users with subtle "string literal" vs. "literal string" differences
in meaning.
It's beneficial for two reasons: it allows us to avoid occasional
PsiWhitespaces at the end of incomplete fragments and also we
can now better report illegal line comments this way, because even though
they still terminate f-strings they are now under the corresponding AST
nodes for expression fragments and can be processed by the annotator
in the same fashion.
