cmTarget: add support for C++ module fileset types

C++ modules have two variants which are of importance to CMake:

  - `CXX_MODULES`: interface modules (those using `export module M;`,
    `export module M:part;`, or `module M:internal_part;`)
  - `CXX_MODULE_HEADER_UNITS`: importable header units

Creating C++ modules or partitions are *not* supported in any other
source listing. This is because the source files must be installed (so
their scope matters), but not part of usage requirements (what it means
for a module source to be injected into a consumer is not clear at this
moment). Due to the way `FILE_SET` works with scopes, they are a perfect
fit as long as `INTERFACE` is not allowed (which it is not).

Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterface.cmake

@@ -0,0 +1,8 @@
+add_library(module-header)
+target_sources(module-header
+  INTERFACE
+    FILE_SET fs TYPE CXX_MODULE_HEADER_UNITS FILES
+      sources/module-header.h)
+target_compile_features(module-header
+  PRIVATE
+    cxx_std_20)