Currently, we deal with escape sequences twice: once when we lex a
string, and a second time when we unescape literals. This PR aims to
remove this duplication, by introducing a new `unescape` mode as a
single source of truth for character escaping rules
When a format string has escaped whitespace characters format
arguments were shifted by one per each escaped character. Account
for these escaped characters when synthesizing the spans.
Fix#55155.
- Point at opening mismatched formatting brace
- Account for differences between raw and regular strings
- Account for differences between the code snippet and `InternedString`
- Add more tests
