patch 8.0.1596: no autocommand specifically for opening a terminal window

Problem:    No autocommand specifically for opening a terminal window.
Solution:   Add TerminalOpen. (?, closes #2484)
diff --git a/src/fileio.c b/src/fileio.c
index 8331c08..05c3df5 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -7773,6 +7773,7 @@
     {"TabEnter",	EVENT_TABENTER},
     {"TabLeave",	EVENT_TABLEAVE},
     {"TermChanged",	EVENT_TERMCHANGED},
+    {"TerminalOpen",	EVENT_TERMINALOPEN},
     {"TermResponse",	EVENT_TERMRESPONSE},
     {"TextChanged",	EVENT_TEXTCHANGED},
     {"TextChangedI",	EVENT_TEXTCHANGEDI},