Add support for CSS grid

[audreytoskin]

I have an HTML document I'm trying to convert to PDF using WeasyPrint. The HTML version makes heavy use of CSS grid styles, and when trying to do the conversion, I get a bunch of warnings about the grid styles being ignored, because of "unknown property" errors.

WARNING: Ignored `display: grid` at 84:3, invalid value.
WARNING: Ignored `grid-column-gap: 1em` at 85:3, unknown property.
WARNING: Ignored `grid-template-columns: 1fr 1fr 1.5fr` at 86:3, unknown property.
WARNING: Ignored `grid-column: 1/4` at 92:3, unknown property.
WARNING: Ignored `display: grid` at 156:3, invalid value.
WARNING: Ignored `grid-gap: 0.5em` at 157:3, unknown property.
WARNING: Ignored `grid-template-columns: 1fr 1fr` at 158:3, unknown property.
WARNING: Ignored `grid-column: 1` at 168:3, unknown property.
WARNING: Ignored `grid-row: 2` at 169:3, unknown property.
[...]

I could be wrong, but I guess this is because tinycss2 doesn't support grid styles yet?

[audreytoskin]

Oh, sorry, I just noticed someone else made the same request in Kozea/WeasyPrint#543 too.

[liZe]

Oh, sorry, I just noticed someone else made the same request in Kozea/WeasyPrint#543 too.

Yes, everything's already done in tinycss2 and cssselect2, the work is needed in WeasyPrint!