Reserved words of rego should not be used as variable or rule name

[vgramer]

What would you like to be added

The following words are reserved and cannot be used as variable names, rule names:

• as
• default
• else
• false
• import
• package
• not
• null
• some
• true
• with

The plugin should show an error if it's used as a variable or rule name.

Implementation details

Don't need to change the grammar, there is an "API" in IDEA to do that