LibWeb: Add a new code generator for CSS enums

Alias values are represented by "alias-name=real-name".

We have a lot of repetitive code for converting between ValueID and
property-specific enums. Let's see if we can generate it. :^)

This first step just produces the enums, from a JSON file. The values in
there are a duplication of what's in Properties.json, but eventually
those will go away.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -36,6 +36,7 @@ set(SOURCES
     CSS/CSSSupportsRule.cpp
     CSS/DefaultStyleSheetSource.cpp
     CSS/Display.cpp
+    CSS/Enums.cpp
     CSS/FontFace.cpp
     CSS/Frequency.cpp
     CSS/Length.cpp
@@ -616,6 +617,16 @@ libweb_js_wrapper(XHR/ProgressEvent)
 libweb_js_wrapper(XHR/XMLHttpRequest)
 libweb_js_wrapper(XHR/XMLHttpRequestEventTarget)
 
+invoke_generator(
+    "Enums"
+    Lagom::GenerateCSSEnums
+    "${CMAKE_CURRENT_SOURCE_DIR}/CSS/Enums.json"
+    ""
+    "CSS/Enums.h"
+    "CSS/Enums.cpp"
+    arguments -j "${CMAKE_CURRENT_SOURCE_DIR}/CSS/Enums.json"
+)
+
 invoke_generator(
     "MediaFeatureID"
     Lagom::GenerateCSSMediaFeatureID