WIP: Psalm v6 + nikic/php-parser v5 compat

[kkmuffme]

php-parser:

• [x] rename the classes e.g. LNumber https://github.com/nikic/PHP-Parser/blob/master/UPGRADE-5.0.md#renamed-nodes
• [x] getLine => getStartLine https://github.com/nikic/PHP-Parser/blob/master/UPGRADE-5.0.md#miscellaneous-changes

Psalm:

• [x] update composer.json for psalm ^5 => ^6

This is a task list to keep track of tasks required to do and is updated continously.