Fix parsing of custom properties with 'var' in value

[TimvdLippe]

(Note: the newline 'changes' are solved after #2484)

The screenshot shows the new behaviour. It only consumes lines with
<css-property-name>:<anything not a ;>var(<anything not a ;>)<optional ; and \n>

• It leaves the first line which was the original issue.
• It consumes the second line, which is a var assignment
• It consumes the third line which is preceded with a var assignment
• It leaves the fourth line, a property between assignments
• It consumes the whole fifth line, which does not have a new line between the two variable assignments
• It consumes the second part of the sixth line, which does not have a new line between the custom property and the variable assignment
• It consumes the first part of the seventh line, which does not have a new line between the variable assignment and the custom property

Fixes #2660

[nazar-pc]

LGTM, 👍

[samccone]

🙇 this is amazing work @JeremybellEU thanks a million!

R: @tjsavage

[tjsavage]

This LGTM. Adding @sorvell to take a look.

[dfreedm]

LGTM