[ENHANCEMENT] Support for RF 7

[d-biehl]

• [x] Native VAR syntax
• [x] Mixed argument support with library keywords
• [x] Tags set globally can be removed using -tag syntax
• [ ] TODO: deprecated things
• [x] tests

see: https://github.com/robotframework/robotframework/blob/master/doc/releasenotes/rf-7.0a1.rst