LibWeb: Parse the border-image-outset property

Author: https://github.com/tcl3
Commit: 98e63e3dd5
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5253
Reviewed-by: https://github.com/AtkinsSJ ✅

Tests/LibWeb/Text/expected/css/CSSStyleProperties-all-supported-properties-and-default-values.txt

@@ -243,6 +243,8 @@ All supported properties and their default values exposed from CSSStylePropertie
 'border-end-end-radius': '0px'
 'borderEndStartRadius': '0px'
 'border-end-start-radius': '0px'
+'borderImageOutset': '0'
+'border-image-outset': '0'
 'borderImageSlice': '100%'
 'border-image-slice': '100%'
 'borderImageSource': 'none'