win: has("wsl") on Windows Subsystem for Linux #7330

Per CMAKE docs, CMAKE_HOST_SYSTEM_VERSION is the result of `uname -r`:
https://cmake.org/cmake/help/v3.4/variable/CMAKE_HOST_SYSTEM_VERSION.html?highlight=uname

    A numeric version string for the system. On systems that support
    uname, this variable is set to the output of uname -r. On other
    systems this is set to major-minor version numbers.

On Windows it is something like "6.1", so it won't match ".*-Microsoft".

Closes #7329

src/nvim/eval.c

@@ -10671,6 +10671,9 @@ static void f_has(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     "windows",
     "winaltkeys",
     "writebackup",
+#if defined(HAVE_WSL)
+    "wsl",
+#endif
     "nvim",
   };