amsmath \dots should do tests after expansion

[davidcarlisle]

As noted in #1448 \dots uses \futurelet testing to determine the next token.

this has lead to multiple additions in recent releases coping with \protected and classic \protect macros, \long macros and finally \if.. tokens, It also has the issue that user-level macros are not expanded so hide any definition which would request a different dots form.

The suggestion is to "f-expand" (\romannumeral) the input stream before doing the tests.
this would expand user commands and allow the removal of most of the \meaning tests checking for macro types.

\DOTSB etc so are all \relax so would be available for testing via a simple delimited macro if the futurelet token is ifx equal to \relax

Code for this wasn't quite ready for the 2024-10 release, but should be made available via a -dev release after that.