Remove ExpressionParser and use the amazing RuboCop::AST::NodePattern

[jonatas]

The RuboCop AST is finally isolated into a separated gem and now we don't need to keep the core of the library as NodePattern is pretty much stable and easy to use.

It also validates syntaxes of the expression and had a slightly different behavior on ... operator.

Basically symbols needs explicitly call with :. nil becomes nil? in most part of the cases and the .something? is just something? now ;)

Kudos to @marcandre for extracting it from RoboCop. My first attempt failed and he simply did it 🚀

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[andriy-baran]

@jonatas Do you plan to merge this?

[jonatas]

Hi @andriy-baran, thank you for showing interest in this! I made it on a weekend and never pushed it again to the upstreaming changes.

I'll not merge it too soon as I need some time to catch up with the latest expression of the rubocop-ast news and guarantee it will be fully compatible. If you have the time to move on, feel free to rebase I'll be happy to review.