LibWeb+WebDriver: Begin processing and matching WebDriver capabilities

Still some TODOs here:
* We don't handle all capabilities (e.g. proxy)
* We don't match the capabilities against the running browser

But this will parse the capabilities JSON object received from the
WebDriver client.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -439,6 +439,7 @@ set(SOURCES
     WebAssembly/WebAssemblyTableConstructor.cpp
     WebAssembly/WebAssemblyTableObject.cpp
     WebAssembly/WebAssemblyTablePrototype.cpp
+    WebDriver/Capabilities.cpp
     WebDriver/Client.cpp
     WebDriver/ElementLocationStrategies.cpp
     WebDriver/Error.cpp