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/feature.h b/src/feature.h
index 138279e..f77ffd6 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -1268,6 +1268,13 @@
 #endif
 
 /*
+ * +terminal		":terminal" command.  Runs a terminal in a window.
+ */
+#if !defined(FEAT_JOB_CHANNEL) && defined(FEAT_TERMINAL)
+# undef FEAT_TERMINAL
+#endif
+
+/*
  * +signs		Allow signs to be displayed to the left of text lines.
  *			Adds the ":sign" command.
  */