patch 9.0.1684: Update libvterm to rev 839

Problem: libvterm slightly outdated
Solution: Update libvterm from rev 818 to rev 839

Notable fix: libvterm now handles DECSM/DECRM with multiple arguents,
so several ncurses programs (e.g. nnn) can enable mouse properly when
run in Vim's terminal in XTerm.

closes: #12746

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
diff --git a/src/libvterm/t/17state_mouse.test b/src/libvterm/t/17state_mouse.test
index e5ba29b..33b7dc2 100644
--- a/src/libvterm/t/17state_mouse.test
+++ b/src/libvterm/t/17state_mouse.test
@@ -55,6 +55,10 @@
   output "\e[M\x60\x36\x2b"
 MOUSEBTN d 5 0
   output "\e[M\x61\x36\x2b"
+MOUSEBTN d 6 0
+  output "\e[M\x62\x36\x2b"
+MOUSEBTN d 7 0
+  output "\e[M\x63\x36\x2b"
 
 !DECRQM on mouse button mode
 PUSH "\e[?1000\$p"
@@ -179,3 +183,9 @@
 MOUSEMOVE 0,0 0
 MOUSEBTN d 1 0
 MOUSEBTN u 1 0
+
+!DECSM can set multiple modes at once
+PUSH "\e[?1002;1006h"
+  settermprop 8 2
+MOUSEBTN d 1 0
+  output "\e[<0;1;1M"