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/29state_fallback.test b/src/libvterm/t/29state_fallback.test
new file mode 100644
index 0000000..adf1c23
--- /dev/null
+++ b/src/libvterm/t/29state_fallback.test
@@ -0,0 +1,19 @@
+INIT
+WANTSTATE f
+RESET
+
+!Unrecognised control 
+PUSH "\x03"
+  control 03
+
+!Unrecognised CSI
+PUSH "\e[?15;2z"
+  csi 0x7a L=3f 15,2
+
+!Unrecognised OSC
+PUSH "\e]27;Something\e\\"
+  osc "27;Something"
+
+!Unrecognised DCS
+PUSH "\ePz123\e\\"
+  dcs "z123"