Catch syntax errors in nested interpolations before Python 3.12

[ntBre]

Summary

This PR fixes the issue I added in #20867 and noticed in #20930. Cases like this
cause an error on any Python version:

f"{1:""}"

which gave me a false sense of security before. Cases like this are still
invalid only before 3.12 and weren't flagged after the changes in #20867:

f'{1: abcd "{'aa'}" }'
           # ^  reused quote
f'{1: abcd "{"\n"}" }'
            # ^  backslash

I didn't recognize these as nested interpolations that also need to be checked
for invalid expressions, so filtering out the whole format spec wasn't quite
right. And elements.interpolations() only iterates over the outermost
interpolations, not the nested ones.

There's basically no code change in this PR, I just moved the existing check
from parse_interpolated_string, which parses the entire string, to
parse_interpolated_element. This kind of seems more natural anyway and avoids
having to try to recursively visit nested elements after the fact in
parse_interpolated_string. So viewing the diff with something like

git diff --color-moved --ignore-space-change --color-moved-ws=allow-indentation-change main

should make this more clear.

Test Plan

New tests

[github-actions]

ruff-ecosystem results

Linter (stable)

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Linter (preview)

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Formatter (stable)

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Formatter (preview)

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