runtime(helptoc): reload cached g:helptoc.shell_prompt when starting toc

runtime(helptoc): reload cached g:helptoc.shell_prompt when starting toc

Follow up on PR 10446 [1] so that changes at run-time (or after loading
a vimrc) are reflected at next use. Instead of "uncaching" the variable
by computing SHELL_PROMPT on each use, which could negatively impact
performance, reload any user settings before creating the TOC.

Also make sure, changes to the shell prompt variable do correctly
invalidate b:toc, so that the table of content is correctly re-created
after user makes any changes.

[1]: https://github.com/vim/vim/pull/10446#issuecomment-2485169333

closes: #16097

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>