os: implement os.is_writable_folder/1

vlib/os/os.v

@@ -543,6 +543,23 @@ pub fn is_executable(path string) bool {
   return C.access(path.str, X_OK) != -1
 }
 
+// `is_writable_folder` - `folder` exists and is writable to the process
+pub fn is_writable_folder(folder string) ?bool {
+	if !os.exists(folder) {
+		return error('`$folder` does not exist')
+	}
+	if !os.is_dir(folder) {
+		return error('`folder` is not a folder')
+	}
+	tmp_perm_check := os.join_path(folder, 'tmp_perm_check')
+	f := os.open_file(tmp_perm_check, 'w+', 0o700) or {
+		return error('cannot write to folder `$folder`: $err')
+	}
+	f.close()
+	os.rm(tmp_perm_check)
+	return true
+}
+
 // `is_writable` returns `true` if `path` is writable.
 pub fn is_writable(path string) bool {
   $if windows {