LibGC+Everywhere: Factor out a LibGC from LibJS

Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:

 * JS::NonnullGCPtr -> GC::Ref
 * JS::GCPtr -> GC::Ptr
 * JS::HeapFunction -> GC::Function
 * JS::CellImpl -> GC::Cell
 * JS::Handle -> GC::Root

Libraries/LibWeb/WebGL/WebGLShaderPrecisionFormat.cpp

@@ -9,7 +9,7 @@
 
 namespace Web::WebGL {
 
-JS_DEFINE_ALLOCATOR(WebGLShaderPrecisionFormat);
+GC_DEFINE_ALLOCATOR(WebGLShaderPrecisionFormat);
 
 WebGLShaderPrecisionFormat::WebGLShaderPrecisionFormat(JS::Realm& realm)
     : WebGLObject(realm)