Add TabNew event

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

src/nvim/ex_docmd.c

@@ -5715,6 +5715,8 @@ void ex_splitview(exarg_T *eap)
     if (win_new_tabpage(cmdmod.tab != 0 ? cmdmod.tab
             : eap->addr_count == 0 ? 0
             : (int)eap->line2 + 1) != FAIL) {
+      apply_autocmds(EVENT_TABNEW, eap->arg, eap->arg,  FALSE, curbuf); 
+      entering_new_tab = true;
       do_exedit(eap, old_curwin);
 
       /* set the alternate buffer for the window we came from */

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},

src/nvim/fileio.h

@@ -95,6 +95,7 @@ typedef enum auto_event {
   EVENT_CURSORMOVEDI,           /* cursor was moved in Insert mode */
   EVENT_TABLEAVE,               /* before leaving a tab page */
   EVENT_TABENTER,               /* after entering a tab page */
+  EVENT_TABNEW,                 /* when creating a new tab */
   EVENT_SHELLCMDPOST,           /* after ":!cmd" */
   EVENT_SHELLFILTERPOST,        /* after ":1,2!cmd", ":w !cmd", ":r !cmd". */
   EVENT_TEXTCHANGED,            /* text was modified */