Add linters to the project

[BassCoder2808]

Linters are tools that help identify and report errors in code, helping to ensure that the codebase is of a high quality and follows best practices. The SPDX Online Tools project could benefit from the addition of linters for Python, JavaScript, HTML, and CSS, which would help to identify and fix issues in the codebase.

Python linters such as Pylint, Flake8, and Pyflakes can help to identify errors, potential bugs, and code smells in Python code. JavaScript linters like ESLint can detect issues such as unused variables, missing semicolons, and incorrect variable scoping. HTML linters like HTMLHint can check for issues such as invalid markup and accessibility errors. Finally, CSS linters like Stylelint can help identify errors such as invalid selectors, unused styles, and syntax errors.

The linters could be integrated into the development process, either through the use of pre-commit hooks or as part of a continuous integration (CI) pipeline. When a developer commits code, the linters would run and report any issues. This would help to catch errors early in the development process, before they become larger problems.