add specs

Author: perazz

doc/specs/stdlib_system.md

@@ -417,3 +417,38 @@ Returns one of the `integer` `OS_*` parameters representing the OS type, from th
 ```fortran
 {!example/system/example_os_type.f90!}
 ```
+
+## `is_directory` - Test if a path is a directory
+
+### Status
+
+Experimental
+
+### Description
+
+This function checks if a specified file system path is a directory. 
+It is designed to work across multiple platforms. On Windows, paths with both forward `/` and backward `\` slashes are accepted.
+
+### Syntax
+
+`result = [[stdlib_io(module):is_directory(function)]] (path)`
+
+### Class
+Function
+
+### Arguments
+
+`path`: Shall be a character string containing the file system path to evaluate. It is an `intent(in)` argument.
+
+### Return values
+
+The function returns a `logical` value:
+
+- `.true.` if the path matches an existing directory.
+- `.false.` otherwise, or if the operating system is unsupported.
+
+### Example
+
+```fortran
+{!example/system/example_is_directory.f90!}
+```