Posting a quoted reply to a message from JS (e.g. using bbs.post_msg) is awkward

The QUOTES.TXT file must exist (when using an external editor configured in SCFG as "All" or "Prompt User"), but we don't expose sbbs_t::quotemsg() in a JS method and don't call it automatically in bbs.post_msg(). This leads to custom message readers needing to do their own QUOTES.TXT file generation, e.g.: https://gitlab.synchro.net/main/sbbs/-/blame/master/xtrn/ecreader/lib.js#L692

Or else risk getting errors like this when trying to post replies via JS:

<Hm Derdoc> !ERROR 2 (No such file or directory) in writemsg.cpp line 396 (writemsg) opening "/sbbs/node1/QUOTES.TXT" access=0

This issue likely impacts other message creation functions too (not just post_msg()).