LibCore: Introduce Core::Environment wrapper

Core::System already had some wrappers for *env() functions, which I've
copied over. But 1) the set of functions there was incomplete, and 2)
the environment feels like an object in its own right, so let's treat it
as one. :^)

Also add `Core::Environment::has(StringView)` for situations where we
just care if a variable is defined.

Userland/Libraries/LibCore/CMakeLists.txt

@@ -8,6 +8,7 @@ set(SOURCES
     DirectoryEntry.cpp
     DirIterator.cpp
     ElapsedTimer.cpp
+    Environment.cpp
     Event.cpp
     EventLoop.cpp
     EventLoopImplementation.cpp