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.

[1]: vim#10446 (comment)

Best-viewed-with: --ignore-space-change

Author: benknoble

runtime/pack/dist/opt/helptoc/autoload/helptoc.vim

@@ -2,9 +2,15 @@ vim9script noclear
 
 # Config {{{1
 
-const SHELL_PROMPT: string = g:
-    ->get('helptoc', {})
-    ->get('shell_prompt', '^\w\+@\w\+:\f\+\$\s')
+var SHELL_PROMPT: string = ''
+
+def UpdateUserSettings() #{{{2
+    SHELL_PROMPT = g:
+        ->get('helptoc', {})
+        ->get('shell_prompt', '^\w\+@\w\+:\f\+\$\s')
+enddef
+
+UpdateUserSettings()
 
 # Init {{{1
 
@@ -200,6 +206,7 @@ enddef
 #}}}1
 # Core {{{1
 def SetToc() #{{{2
+    UpdateUserSettings()
     var toc: dict<any> = {entries: []}
     var type: string = GetType()
     toc.changedtick = b:changedtick