feat: allow output to nested directory

[jackchuka]

This pull request improves the reliability of file output operations in the mdschema CLI by ensuring that any necessary directories are created before writing output files. This prevents errors when the specified output path includes directories that do not yet exist.

Key improvements by theme:

Output file handling enhancements:

• Before writing to an output file, the code now checks if the parent directory exists and creates it if necessary in derive.go, generate.go, and schema.go using os.MkdirAll with appropriate permissions. [1] [2] [3]
• Added imports for the filepath package in all three command files to support directory path operations. [1] [2] [3]