Add "showdefinition" to Vim syntax highlight

This is a MacVim-specific function.

Author: ychin

runtime/syntax/vim.vim

@@ -161,6 +161,8 @@ syn keyword vimFuncName contained reg_recording reltime reltimefloat reltimestr
 syn keyword vimFuncName contained sort sound_clear sound_playevent sound_playfile sound_stop soundfold spellbadword spellsuggest split sqrt srand state str2blob str2float str2list str2nr strcharlen strcharpart strchars strdisplaywidth strftime strgetchar stridx string strlen strpart strptime strridx strtrans strutf16len strwidth submatch substitute swapfilelist swapinfo swapname synID synIDattr synIDtrans synconcealed synstack system systemlist tabpagebuflist tabpagenr tabpagewinnr tagfiles taglist tan tanh tempname term_dumpdiff term_dumpload term_dumpwrite term_getaltscreen term_getansicolors term_getattr term_getcursor term_getjob term_getline term_getscrolled term_getsize term_getstatus term_gettitle term_gettty term_list term_scrape term_sendkeys term_setansicolors
 syn keyword vimFuncName contained term_setapi term_setkill term_setrestore term_setsize term_start term_wait terminalprops test_alloc_fail test_autochdir test_feedinput test_garbagecollect_now test_garbagecollect_soon test_getvalue test_gui_event test_ignore_error test_mswin_event test_null_blob test_null_channel test_null_dict test_null_function test_null_job test_null_list test_null_partial test_null_string test_null_tuple test_option_not_set test_override test_refcount test_setmouse test_settime test_srand_seed test_unknown test_void timer_info timer_pause timer_start timer_stop timer_stopall tolower toupper tr trim trunc tuple2list type typename undofile undotree uniq uri_decode uri_encode utf16idx values virtcol virtcol2col visualmode wildmenumode wildtrigger
 syn keyword vimFuncName contained win_execute win_findbuf win_getid win_gettype win_gotoid win_id2tabwin win_id2win win_move_separator win_move_statusline win_screenpos win_splitmove winbufnr wincol windowsversion winheight winlayout winline winnr winrestcmd winrestview winsaveview winwidth wordcount writefile xor
+" MacVim specific
+syn keyword vimFuncName contained showdefinition
 
 " Predefined variable names {{{2
 " GEN_SYN_VIM: vimVarName, START_STR='syn keyword vimVimVarName contained', END_STR=''