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.
Also fixed the problem that whitespaces inside literal parts of f-strings were
matched as PsiWhitespaces instead of FSTRING_TEXT tokens and updated all relevant
test data.
When the terminating quote is either at the beginning of a string literal or
inside it, either in the given f-string itself or inside a nested one and
either in normal fragment or in a fragment located inside a format part of
another fragment. Normal string literals and nested f-strings are also checked
separately.
Mikhail Golubev