[libc] add wctype.h header

[Prabhuk]

Add basic configurations to generate wctype.h header file. To begin with
this header file just exposes one function iswalpha.

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff HEAD~1 HEAD --extensions h,cpp -- libc/src/wctype/iswalpha.cpp libc/src/wctype/iswalpha.h

View the diff from clang-format here.

diff --git a/libc/src/wctype/iswalpha.cpp b/libc/src/wctype/iswalpha.cpp
index 920ce70e2..c8e746416 100644
--- a/libc/src/wctype/iswalpha.cpp
+++ b/libc/src/wctype/iswalpha.cpp
@@ -14,8 +14,6 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
-LLVM_LIBC_FUNCTION(bool, iswalpha, (wint_t c)) {
-  return internal::iswalpha(c);
-}
+LLVM_LIBC_FUNCTION(bool, iswalpha, (wint_t c)) { return internal::iswalpha(c); }
 
 } // namespace LIBC_NAMESPACE_DECL