Update PR template

[olegbespalov]

What?

Simplifies and add structure to the pull requests template.

Why?

Having checklists is a common practice where we can explicitly ask (or later refer) contributors/maintainers if they have done some essential things like (self-review, linters, and test runs).

[codecov-commenter]

Codecov Report

Merging #3131 (35ce431) into master (a0c43b8) will increase coverage by 0.03%.
The diff coverage is 71.42%.

❗ Current head 35ce431 differs from pull request most recent head 0bacfff. Consider uploading reports for the commit 0bacfff to get more accurate results

@@            Coverage Diff             @@
##           master    #3131      +/-   ##
==========================================
+ Coverage   73.86%   73.89%   +0.03%     
==========================================
  Files         243      243              
  Lines       18492    18488       -4     
==========================================
+ Hits        13659    13662       +3     
+ Misses       3964     3960       -4     
+ Partials      869      866       -3     

Flag	Coverage Δ
ubuntu	73.83% <71.42%> (+0.03%)	⬆️
windows	73.72% <71.42%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cloudapi/config.go	89.47% <ø> (ø)
lib/consts/consts.go	78.57% <ø> (ø)
output/cloud/expv2/metrics_client.go	59.37% <50.00%> (+3.81%)	⬆️
output/cloud/expv2/flush.go	84.61% <100.00%> (ø)
output/cloud/expv2/output.go	86.84% <100.00%> (ø)

... and 3 files with indirect coverage changes

[codebien]

Good idea, thanks for taking care of it.

[codebien]

contributors/maintainers if they have done some essential things like (self-review, linters, and test runs).

@olegbespalov can you set clearly the expectations about it? Do you expect us as reviewers will add and check all of them?

[olegbespalov]

Hey @codebien .

Do you expect us as reviewers will add and check all of them?

TL;DR: No, I don't expect any reviewers to mark checkbox items.

This checklist is, first of all, for the casual k6's contributors. The idea is to help quickly figure out what is necessary to check even before the code review starts. But I believe it helps also non-casual contributors, even us as maintainers. So the pull request creator should mark the checkboxes, that are applicable to the current case context.