Add ifshort linter

[esimonov]

ifshort is a linter for checking that the short syntax for if-statements is used whenever possible. For example, it will tell that the variables ok and err are unnecessarily defined outside the scope of the if-statements - the only places where they are referred to:

func someFunc(k string, m map[string]interface{}) {
	_, ok := m[k]
	if !ok { // could be `if _, ok := m[k]; !ok`
		return
	}

	err := otherFunc1()
	if err != nil { // could be `if err := otherFunc1(); err != nil`
		otherFunc2(err)
	}
}

At the same time, it won't complain about the variables that actually require to be present in a wider scope.

https://github.com/esimonov/ifshort

[boring-cyborg]

Hey, thank you for opening your first Pull Request !

[CLAassistant]

All committers have signed the CLA.

[ldez]

Hello,

have you thought about using tag/version?

There are several advantages:

• it's more "Go module idiomatic"
• that allow the dependabot to update automatically golangci-lint when you release a new version

[esimonov]

@ldez Thank you for your suggestion. I updated my PR.

[ldez]

LGTM

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