patch 8.1.1111: it is not easy to check for infinity

Problem:    It is not easy to check for infinity.
Solution:   Add isinf(). (Ozaki Kiichi, closes #3787)

runtime/doc/eval.txt

@@ -2411,6 +2411,8 @@ inputsecret({prompt} [, {text}]) String	like input() but hiding the text
 insert({object}, {item} [, {idx}]) List	insert {item} in {object} [before {idx}]
 invert({expr})			Number	bitwise invert
 isdirectory({directory})	Number	|TRUE| if {directory} is a directory
+isinf({expr})			Number	determine if {expr} is infinity value
+					(positive or negative)
 islocked({expr})		Number	|TRUE| if {expr} is locked
 isnan({expr})			Number	|TRUE| if {expr} is NaN
 items({dict})			List	key-value pairs in {dict}
@@ -5772,6 +5774,16 @@ isdirectory({directory})				*isdirectory()*
 		exist, or isn't a directory, the result is |FALSE|.  {directory}
 		is any expression, which is used as a String.
 
+isinf({expr})						*isinf()*
+		Return 1 if {expr} is a positive infinity, or -1 a negative
+		infinity, otherwise 0. >
+			:echo isinf(1.0 / 0.0)
+<			1 >
+			:echo isinf(-1.0 / 0.0)
+<			-1
+
+		{only available when compiled with the |+float| feature}
+
 islocked({expr})					*islocked()* *E786*
 		The result is a Number, which is |TRUE| when {expr} is the
 		name of a locked variable.