feat: add loopvar linter

[karamaru-alpha]

I'd like to add https://github.com/karamaru-alpha/loopvar.
This linter detects places where loop variables are copied.

For Go 1.21(set GOEXPERIMENT=loopvar) and Go 1.22~, it is unnecessary to copy loop variables because these variables have per-iteration scope instead of per-loop scope.

I understand that introducing this linter might be a bit premature, but I added it thinking it will be useful in the future.

cf. Fixing For Loops in Go 1.22

For Go 1.22, we plan to change for loops to make these variables have per-iteration scope instead of per-loop scope.

Go 1.21 includes a preview of the scoping change. If you compile your code with GOEXPERIMENT=loopvar set in your environment, then the new semantics are applied to all loops.

golangci-lint run --no-config --disable-all -E loopvar ./test/testdata/loopvar.go

test/testdata/loopvar.go:12:3: The loop variable "i" should not be copied (Go 1.22~ or Go 1.21 GOEXPERIMENT=loopvar) (loopvar)
		i := i // want `The loop variable "i" should not be copied \(Go 1.22~ or Go 1.21 GOEXPERIMENT=loopvar\)`
		^
test/testdata/loopvar.go:16:3: The loop variable "v" should not be copied (Go 1.22~ or Go 1.21 GOEXPERIMENT=loopvar) (loopvar)
		_v := v // want `The loop variable "v" should not be copied \(Go 1.22~ or Go 1.21 GOEXPERIMENT=loopvar\)`
		^
test/testdata/loopvar.go:30:3: The loop variable "i" should not be copied (Go 1.22~ or Go 1.21 GOEXPERIMENT=loopvar) (loopvar)
		i := i // want `The loop variable "i" should not be copied \(Go 1.22~ or Go 1.21 GOEXPERIMENT=loopvar\)`

[boring-cyborg]

Hey, thank you for opening your first Pull Request !

[CLAassistant]

All committers have signed the CLA.

[ldez]

In order for a pull request adding a linter to be reviewed, the linter and the PR must follow some requirements.

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Pull Request Description

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.golangci.reference.yml

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Recommendations

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---

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[Antonboom]

Antagonist of #1404.
Naming loopvar is confusing.

[karamaru-alpha]

@Antonboom
Thank you for the valuable information!
Is #1404 scheduled to be introduced? If so, it may be necessary to rename this linter to something like "antilooppointer".