patch 8.2.1535: it is not possible to specify cell widths of characters

Problem:    It is not possible to specify cell widths of characters.
Solution:   Add setcellwidths().

runtime/doc/eval.txt

@@ -2768,6 +2768,7 @@ setbufline({expr}, {lnum}, {text})
 					{expr}
 setbufvar({expr}, {varname}, {val})
 				none	set {varname} in buffer {expr} to {val}
+setcellwidths({list})		none	set character cell width overrides
 setcharsearch({dict})		Dict	set character search from {dict}
 setcmdpos({pos})		Number	set cursor position in command-line
 setenv({name}, {val})		none	set environment variable
@@ -8937,6 +8938,29 @@ setbufvar({expr}, {varname}, {val})			*setbufvar()*
 		third argument: >
 			GetValue()->setbufvar(buf, varname)
 
+
+setcellwidths({list})					*setcellwidths()*
+		Specify overrides for cell widths of character ranges.  This
+		tells Vim how wide characters are, counted in screen cells.
+		This overrides 'ambiwidth'.  Example: >
+		   setcellwidths([[0xad, 0xad, 1],
+		   		\ [0x2194, 0x2199, 2]]
+
+<					*E1109* *E1110* *E1111* *E1112* *E1113*
+		The {list} argument is a list of lists with each three
+		numbers. These three numbers are [low, high, width].  "low"
+		and "high" can be the same, in which case this refers to one
+		character. Otherwise it is the range of characters from "low"
+		to "high" (inclusive).  "width" is either 1 or 2, indicating
+		the character width in screen cells.
+		An error is given if the argument is invalid, also when a
+		range overlaps with another.
+		Only characters with value 0x100 and higher can be used.
+
+		To clear the overrides pass an empty list: >
+		   setcellwidths([]);
+
+
 setcharsearch({dict})					*setcharsearch()*
 		Set the current character search information to {dict},
 		which contains one or more of the following entries:

runtime/doc/options.txt

@@ -701,7 +701,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 	"double":	Use twice the width of ASCII characters.
 							*E834* *E835*
 	The value "double" cannot be used if 'listchars' or 'fillchars'
-	contains a character that would be double width.
+
+	The values are overruled for characters specified with
+	|setcellwidths()|.
 
 	There are a number of CJK fonts for which the width of glyphs for
 	those characters are solely based on how many octets they take in

runtime/doc/usr_41.txt

@@ -611,6 +611,7 @@ String manipulation:					*string-functions*
 	strchars()		length of a string in characters
 	strwidth()		size of string when displayed
 	strdisplaywidth()	size of string when displayed, deals with tabs
+	setcellwidths()		set character cell width overrides
 	substitute()		substitute a pattern match with a string
 	submatch()		get a specific match in ":s" and substitute()
 	strpart()		get part of a string using byte index