Linter: Make it possible to configure linter rules in the config file on per folder basis

Our project has a unit testing directory, where tests are calling private functions (they start with `_`). For tests code the linter reports errors, that the private functions are called, but for our project we want to ignore them.

There only way to configure the linter I found from the docs is with [comments](https://github.com/GDQuest/GDScript-formatter#ignoring-lines) and [cli args](https://github.com/GDQuest/GDScript-formatter#disabling-specific-rules), which is not scalable for the team or the whole bunch of unit tests files with hundreds of calls.

Start by reading the linked documentation on ignoring lines and disabling specific rules, then inspect how the linter currently handles comments and CLI arguments. Define how rules should be overridden for the unit testing directory and verify that private-function calls in the tests no longer produce errors without per-line comments.

Written by the indexing model from the issue text.