Rename a directory source/slang-stdlib to slang-core-module (#5417)

The directory name of `source/slang-stdlib` is changed to `source/slang-core-module`.
We will use the term "core module" instead of "standard library" from now on.

Author: jkwak-work

CMakeLists.txt

@@ -262,7 +262,7 @@ add_subdirectory(source/slang-wasm)
 add_subdirectory(source/slang-glslang)
 add_subdirectory(tools)
 add_subdirectory(prelude)
-add_subdirectory(source/slang-stdlib)
+add_subdirectory(source/slang-core-module)
 add_subdirectory(source/slang)
 add_subdirectory(source/slangc)
 add_subdirectory(examples)