runtime(vim): Update base-syntax, add legacy header folding

Allow for syntax-based folding of Vim9 script legacy header regions.

This is enabled with the "H" flag of the g:vimsyn_folding config variable.

closes: #14530

Signed-off-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index cc04c59..0688575 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -1,4 +1,4 @@
-*syntax.txt*	For Vim version 9.1.  Last change: 2024 Apr 09
+*syntax.txt*	For Vim version 9.1.  Last change: 2024 Apr 13
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -3764,12 +3764,15 @@
    g:vimsyn_folding == 0 or doesn't exist: no syntax-based folding
    g:vimsyn_folding =~ 'a' : augroups
    g:vimsyn_folding =~ 'f' : fold functions
+   g:vimsyn_folding =~ 'h' : fold heredocs
+   g:vimsyn_folding =~ 'H' : fold Vim9-script legacy headers
    g:vimsyn_folding =~ 'l' : fold lua      script
    g:vimsyn_folding =~ 'm' : fold mzscheme script
    g:vimsyn_folding =~ 'p' : fold perl     script
    g:vimsyn_folding =~ 'P' : fold python   script
    g:vimsyn_folding =~ 'r' : fold ruby     script
    g:vimsyn_folding =~ 't' : fold tcl      script
+let g:vimsyn_folding = 'acfhlmpPrt'
 <
 							*g:vimsyn_noerror*
 Not all error highlighting that syntax/vim.vim does may be correct; Vim script