feat(css): Update syntax for `content` property

[skyclouds2001]

Description

update content property: add attr() update <content-list> syntax: add attr(); remove contents leader() and <target> which are not supported and non-standard at all

note that spec defines as [ <string> | <image> | <attr()> | contents | <quote> | <leader()> | <target> | <string()> | <content()> | <counter> ]+, but many are not-supported so not added at present

Motivation

Additional details

https://developer.mozilla.org/en-US/docs/Web/CSS/content https://drafts.csswg.org/css-content/#propdef-content https://drafts.csswg.org/css-content/#typedef-content-content-list

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