Utilities/Sphinx: Allow explicit target for genex

Split the genex directive into its own class, allowing a slight
simplification of CMakeObject. Add ability to specify an explicit target
name for the same.

Use this to provide a target for the `$<TARGET_PROPERTY:prop>` generator
expression which is otherwise missing one (due to overlap with
`$<TARGET_PROPERTY:tgt,prop>`).  With this one can write:

    :genex:`$<TARGET_PROPERTY:prop> <TARGET_PROPERTY:prop>`

to link the second variant.

Fixes: #24573

Help/dev/documentation.rst

@@ -241,6 +241,10 @@ Document a "genex" object:
 
 The directive requires a single argument, the generator expression name.
 
+The optional ``:target:`` option allows a custom target name to be specified.
+Because this will affect the ability to reference the "genex" object using the
+``:genex:`` role, this option should be used very sparingly.
+
 ``signature`` directive
 ^^^^^^^^^^^^^^^^^^^^^^^