Ladybird: Move helper processes to CMAKE_INSTALL_LIBEXECDIR

It aligns better with the Filesystem Heirarchy Standard[1] to put our
program-specific helper programs that are not intended to be executed by
the user of the application in $prefix/libexec or in whatever the
packager sets as the CMake equivalent. Namely, on Debian systems this
should be /usr/lib/Ladybird or similar.

[1] https://refspecs.linuxfoundation.org/FHS_3.0/fhs-3.0.html#usrlibexec

Meta/gn/secondary/Ladybird/WebContent/BUILD.gn

@@ -81,4 +81,5 @@ executable("WebContent") {
     cflags_cc = [ "-DHAS_ACCELERATED_GRAPHICS" ]
     deps += [ "//Userland/Libraries/LibAccelGfx" ]
   }
+  output_dir = "$root_out_dir/libexec"
 }