Support hask mark next to curly braces in CSS grammar

[tidoust]

The CSS Backgrounds Level 4 spec currently defines the value of box-shadow-offset as:

none | <length>{2}#

(the latter part to mean a "comma-separated list of horizontal and vertical offset pairs")

CSSTree's definitionSyntax considers that to be invalid. I suppose that the problem is the use of # next to {2}. Creating a group solves the parsing issue:

none | [<length>{2}]#

The hash-mark-next-to-curly-braces construct is not explicitly accounted for in the CSS Values spec for now, but grouping brackets tend to be skipped in CSS specs when possible, and this construct does not seem ambiguous.