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Problem: libvterm slightly outdated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/Makefile b/src/libvterm/Makefile
index ba4a315..94ca409 100644
--- a/src/libvterm/Makefile
+++ b/src/libvterm/Makefile
@@ -36,14 +36,11 @@
 
 HFILES_INT=$(sort $(wildcard src/*.h)) $(HFILES)
 
-VERSION_MAJOR=0
-VERSION_MINOR=3
-
 VERSION_CURRENT=0
 VERSION_REVISION=0
 VERSION_AGE=0
 
-VERSION=$(VERSION_MAJOR).$(VERSION_MINOR)
+VERSION=0.3.3
 
 PREFIX=/usr/local
 BINDIR=$(PREFIX)/bin