[Python] Fix string replacement fields (#3797)

* [Python] Fix string replacement fields

Fixes #3649

This commit closely implements python's format string syntax.

see: https://docs.python.org/3.11/library/string.html#format-string-syntax

It means, replacement field names must be valid qualified identifiers, even
though python syntactically resolves all kinds of key names.

Replacement field termination is handled separately for each kind of quotation.

* [Python] Refactor f-strings

This commit ...

1. aligns implementation of f-string replacement fields with normal string
   replacements.

2. implements f-string replacements for each kind of quotation in order to
   correctly bailout at EOL or terminating quotation mark type.

   Note: Nested quotes are supported in replacement expressions only as of
         python 3.12. They keep unsupported in format-specs.

3. moves escaped braces back into f-string-replacements context and adjusts
   its include positions (moves them before normal `escaped-chars`).

4. removes `meta.format-spec` from `:` to align scoping with replacement fields
   in normal strings.

5. adds some tests to verify syntax being prepared for python 3.12