patch 8.0.0693: no terminal emulator support

Problem:    No terminal emulator support.  Cannot properly run commands in the
            GUI.  Cannot run a job interactively with an ssh connection.
Solution:   Very early implementation of the :terminal command.  Includes
            libvterm converted to ANSI C.  Many parts still missing.
diff --git a/src/libvterm/t/92lp1640917.test b/src/libvterm/t/92lp1640917.test
new file mode 100644
index 0000000..70de439
--- /dev/null
+++ b/src/libvterm/t/92lp1640917.test
@@ -0,0 +1,13 @@
+INIT
+WANTSTATE 
+
+!Mouse reporting should not break by idempotent DECSM 1002
+PUSH "\e[?1002h"
+MOUSEMOVE 0,0 0
+MOUSEBTN d 1 0
+  output "\e[M\x20\x21\x21"
+MOUSEMOVE 1,0 0
+  output "\e[M\x40\x21\x22"
+PUSH "\e[?1002h"
+MOUSEMOVE 2,0 0
+  output "\e[M\x40\x21\x23"