Add optional rule to prevent empty lines at the start of methods or closures

[Bersaelor]

So far we have vertical_whitespace with the max_empty_lines parameter.

There are plenty of places where empty lines are good to increase grouping.

One thing we have observed though and would like to prevent in our projects are empty lines at the beginning, like in

func startDoingStuff() -> Result {

    print("Now doing stuff")
    // ...

    return someResult
}

or

[0...1].map { number in

    print("number: \(number)")
    // ...
    return number* number
}

we don't think there is any reason to allow empty lines at the start of methods so I would like to propose a new rule disallowing this.

I'm about to make a PR too, but since I haven't used sourcery yet I might need a few minutes to get it right (and make all tests pass).

[marcelofabri]

Related to #1345 and #640

[Bersaelor]

Thank you for linking those issues @marcelofabri .
I agree for type declarations a padding at the start or end is something many people do.
What we wanted to prevent in our team is code being needlessly bloated by empty lines at the beginning of func or trailing closures.
Not 100% sure how to separate the type declarations from the closure implementations with regex, will try my best.
Example:

        nonTriggeringExamples: [
            "guard true else { return true }",
            "guard true else {\n return true\n}",
            "if true else {\n return true\n}",
            "method(name: String) {\n return name\n}",
            "method(name: String) { return name }",
            "[1, 2].map { $0 + 1 }",
            "[1, 2].map {\n $0 + 1\n}",
            "[1, 2].map { number in\n return number + 1 \n}"
            "struct AwesomeStruct {\n\n let awesomeName: String",
            "enum AmazingFood: String {\n\n case cheese",
            "class CoolClass {\n\n var lastOpened: Date"
        ],
        triggeringExamples: [
            "guard true else {\n↓\n return true\n}",
            "if true else {\n↓\n return true\n}",
            "method(name: String) {\n↓\n return name\n}",
            "[1, 2].map {\n↓\n $0 + 1\n}",
            "[1, 2].map { number in\n↓\n return number + 1 \n}"
        ]

WiP branch: https://github.com/Bersaelor/SwiftLint/tree/feature/NoBlankFirstClosureLineRule