Add code linter and coding standard check

[nuest]

• Add instructions to README
• Link to instructions from CONTRIBUTING.md
• Run linter as an independent GitHub Action (with a pre-commit GitHub Hook)
• Linter should also verify compliance with the coding standards
• Linter should have good integration with VS Code

Starting points:

• built-in linter (php -l <file-name>)
• https://web20university.com/posts/php-linting/
• https://code-basics.com/languages/php/lessons/linting
• https://psalm.dev/

This is a follow-up to #3 where we require contributors to make sure code lints.

[dxL1nus]

I decided on the VS Code plugin PHP Intelephense and the following example GitHub Hook pre-commit script provided by https://web20university.com/posts/php-linting/ :

#!/bin/sh
files=$(git diff --cached --name-only --diff-filter=ACM | grep '.php$')
if [ "$files" = "" ]; then 
    exit 0 
fi

for file in $files
do
    php -l "$file"
    if [ $? -ne 0 ]; then
        exit 1
    fi
done