Add missing GCPM functions

[Mouvedia]

https://www.antenna.co.jp/AHF/help/en/ahf-css6.html#css3-functions
https://www.w3.org/TR/css-gcpm-3/

• content()
• string()
• running()

related: stylelint/stylelint#8172

[niksy]

PR welcome, if it can’t be found on MDN, you can add it to list of experimental functions.