Added test coverage for plugin type attribute, added checkstyle suppression mechanism

[VitaliyBoyko]

Description (*)

• Covered plugin type XML attribute completion and reference navigation by functional tests.
• Added configs that allows to suppress warnings in the checkstyle checker.
• Suppressed warnings in com.magento.idea.magento2plugin.completion.xml.XmlCompletionContributor and com.magento.idea.magento2plugin.reference.xml.XmlReferenceContributor as files requires decomposition, performing of it will be possible after full test coverage of completion and reference features.
• Fixed issue with plugin type completion

Fixed Issues (if relevant)

1. Test coverage. Cover plugin type attribute completion and reference in the di.xml by tests #133: Test coverage. Cover plugin type attribute completion and reference in the di.xml by tests

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with integration/functional tests (if applicable)
• All automated tests passed successfully (all builds are green)

[coderimus]

@VitaliyBoyko thank you for the work well done! 👍