CSS: Add flex-grow and flex-shrink to cssNumber

Fixes #14888
Closes gh-1536

.mailmap

@@ -77,6 +77,7 @@ Richard D. Worth <rdworth@gmail.com>
 Rick Waldron <waldron.rick@gmail.com>
 Rick Waldron <waldron.rick@gmail.com> <rick@bocoup.com>
 Robert Katić <robert.katic@gmail.com>
+Roman Reiß <me@silverwind.io>
 Ron Otten <r.j.g.otten@gmail.com>
 Sai Lung Wong <sai.wong@huffingtonpost.com>
 Scott González <scott.gonzalez@gmail.com> <sgonzale@sgonzale-laptop.local>

AUTHORS.txt

@@ -216,3 +216,4 @@ Jakob Stoeck <jakob@pokermania.de>
 Christopher Jones <chris@cjqed.com>
 Forbes Lindesay <forbes@lindesay.co.uk>
 S. Andrew Sheppard <andrew@wq.io>
+Roman Reiß <me@silverwind.io>

src/css.js

@@ -222,6 +222,8 @@ jQuery.extend({
 	cssNumber: {
 		"columnCount": true,
 		"fillOpacity": true,
+		"flexGrow": true,
+		"flexShrink": true,
 		"fontWeight": true,
 		"lineHeight": true,
 		"opacity": true,