Add CSS filter() function

Fixes #3378

This has been supported since Safari 9, see https://caniuse.com/css-filter-function.

Author: nt1m

features/filter-function.yml

@@ -0,0 +1,7 @@
+name: filter()
+description: The `filter()` CSS function applies one or more graphic effects to a CSS image, such as a background image. You can use functions, such as `blur()` and `drop-shadow()`, alone or combined to produce different effects. For instance, `background: filter(url(image.png), blur(2px))`.
+spec: https://drafts.csswg.org/filter-effects-1/#FilterCSSImageValue
+group: css
+caniuse: css-filter-function
+compat_features:
+  - css.types.image.filter

features/filter-function.yml.dist

@@ -0,0 +1,10 @@
+# Generated from: filter-function.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support:
+    safari: "9.1"
+    safari_ios: "9.3"
+compat_features:
+  - css.types.image.filter

features/filter.yml

@@ -1,5 +1,5 @@
 name: filter
-description: The `filter` CSS property applies one or more graphic effects to an element. You can use filter functions, such as `blur()` and `drop-shadow()`, alone or combined to produce different effects.
+description: The `filter` CSS property applies one or more graphic effects to an element. You can use filter functions, such as `blur()` and `drop-shadow()`, alone or combined to produce different effects. For instance, `filter: blur(2px)`.
 spec: https://drafts.csswg.org/filter-effects-1/#propdef-filter
 group: css
 caniuse: css-filters