Add \nsubseteq LaTeX symbol to Dr. Racket

[rxg]

\nsubseteq would be well-mapped to Unicode U+2288 as per:
https://www.fileformat.info/info/unicode/char/2288/index.htm

and

https://www.tutorialspoint.com/tex_commands/nsubseteq.htm

At the same time, \subsetneq would be well-mapped to Unicode U+228A as per:

https://www.fileformat.info/info/unicode/char/228a/index.htm
and
https://www.tutorialspoint.com/tex_commands/subsetneq.htm

[sorawee]

You can submit a PR to add an entry to https://github.com/racket/gui/blob/master/tex-table/tex-table.rkt.

[rfindler]

It turns out that a change is typically needed to this function (or somewhere nearby) for these changes. That's because DrRacket includes a list of all of them in its manual which gets built to PDF.