Add TabNew event

TabNew triggers when entering a new tab page, but not when entering an
already created one.

src/nvim/fileio.c

@@ -5247,6 +5247,7 @@ static struct event_name {
   {"Syntax",          EVENT_SYNTAX},
   {"TabEnter",        EVENT_TABENTER},
   {"TabLeave",        EVENT_TABLEAVE},
+  {"TabNew",          EVENT_TABNEW},
   {"TermChanged",     EVENT_TERMCHANGED},
   {"TermResponse",    EVENT_TERMRESPONSE},
   {"TextChanged",     EVENT_TEXTCHANGED},