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Problem: libvterm slightly outdated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/30state_pen.test b/src/libvterm/t/30state_pen.test
index 92cf01d..1a6cd01 100644
--- a/src/libvterm/t/30state_pen.test
+++ b/src/libvterm/t/30state_pen.test
@@ -123,3 +123,11 @@
 PUSH "\e[75m"
   ?pen small = off
   ?pen baseline = normal
+
+!DECSTR resets pen attributes
+PUSH "\e[1;4m"
+  ?pen bold = on
+  ?pen underline = 1
+PUSH "\e[!p"
+  ?pen bold = off
+  ?pen underline = 0