Auto merge of #18336 - upsuper:two-stage-at-rule, r=SimonSapin

Parse at-rule without block in two stages

This is the Servo side change necessary for servo/rust-cssparser#180.

----

This also pulls in other changes from cssparser 0.20.0.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18336)
<!-- Reviewable:end -->

tests/unit/gfx/Cargo.toml

@@ -10,7 +10,7 @@ path = "lib.rs"
 doctest = false
 
 [dependencies]
-cssparser = "0.19"
+cssparser = "0.20"
 gfx = {path = "../../../components/gfx"}
 ipc-channel = "0.8"
 style = {path = "../../../components/style"}

tests/unit/style/Cargo.toml

@@ -12,7 +12,7 @@ doctest = false
 [dependencies]
 byteorder = "1.0"
 app_units = "0.5"
-cssparser = "0.19"
+cssparser = "0.20"
 euclid = "0.15"
 html5ever = "0.19"
 parking_lot = "0.4"

tests/unit/stylo/Cargo.toml

@@ -13,7 +13,7 @@ doctest = false
 
 [dependencies]
 atomic_refcell = "0.1"
-cssparser = "0.19"
+cssparser = "0.20"
 env_logger = "0.4"
 euclid = "0.15"
 geckoservo = {path = "../../../ports/geckolib"}