eval: add wait()

closes #10362

runtime/doc/eval.txt

@@ -2427,6 +2427,8 @@ uniq({list} [, {func} [, {dict}]])
 values({dict})			List	values in {dict}
 virtcol({expr})			Number	screen column of cursor or mark
 visualmode([expr])		String	last visual mode used
+wait({timeout}, {condition}[, {interval}])
+				Number  Wait until {condition} is satisfied
 wildmenumode()			Number	whether 'wildmenu' mode is active
 win_findbuf({bufnr})		List	find windows containing {bufnr}
 win_getid([{win} [, {tab}]])	Number	get |window-ID| for {win} in {tab}
@@ -8858,6 +8860,25 @@ visualmode([expr])						*visualmode()*
 		a non-empty String, then the Visual mode will be cleared and
 		the old value is returned.  See |non-zero-arg|.
 
+wait({timeout}, {condition}[, {interval}])			*wait()*
+		Wait until {condition} is satisfied, where {condition} is a
+		|Funcref| or a |string| containing an expression.
+
+		{timeout} is the maximum number of milliseconds to wait,
+        -1 means forever.
+
+		By default, the condition is evaluated on user and internal
+		events. If {interval} is given, the condition is evaluated
+		every {interval} milliseconds in addition. This can be useful
+		to guarantee that the function returns when the condition is
+		satisfied even if the editor is idle.
+
+		Returns one of the following:
+			*  0 if the condition was satisfied before the timeout
+			* -1 if the timeout was exceeded
+			* -2 if the function was interrupted
+			* -3 if an error occurred
+
 wildmenumode()					*wildmenumode()*
 		Returns |TRUE| when the wildmenu is active and |FALSE|
 		otherwise.  See 'wildmenu' and 'wildmode'.

runtime/doc/usr_41.txt

@@ -959,6 +959,7 @@ Timers:						*timer-functions*
 	timer_stop()		stop a timer
 	timer_stopall()		stop all timers
 	timer_info()		get information about timers
+	wait()			wait for a condition
 
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 	taglist()		get list of matching tags