add 'predeclared' linter

[nishanths]

https://github.com/nishanths/predeclared

This adds the predeclared linter, which reports code that shadows one of Go's predeclared identifiers.

[CLAassistant]

All committers have signed the CLA.

[nishanths]

Fixed the tests. This should be ready for review.

[iwankgb]

@golangci/team is anyone opposed to adding this linter?

[Nivl]

@golangci/team is anyone opposed to adding this linter?

I'm not. I actually like it a lot.

[magnetikonline]

Yep, agree too. TIL this was actually possible and this is valid Go code:

package main

import "fmt"

func main() {
	fmt.Println("Hello")
	append()
}

func append() {
	fmt.Println("Hello")
}

I'd almost argue this should be a default linter!

[nishanths]

Thanks for reviewing. It looks like I have the ability to merge this pull request. Shall I go ahead, or should I wait for a maintainer to merge this/more reviews?