Add self update

[rafael-vitor]

Summary

closes: #1139
Putting back yarn self-update as we now have a way of knowing which installation method was used and how to properly update.

Test plan

As this is my first contribution to this project I'd really appreciate some feedback before going into implementing tests. I'll add them asap.

I'm looking forward for some feedback on this! Thanks!

[buildsize]

File name	Previous Size	New Size	Change
yarn-[version].noarch.rpm	1.1 MB	1.11 MB	2.35 KB (0%)
yarn-[version].js	4.47 MB	4.47 MB	3.05 KB (0%)
yarn-legacy-[version].js	4.66 MB	4.66 MB	3.05 KB (0%)
yarn-v[version].tar.gz	1.12 MB	1.12 MB	215 bytes (0%)
yarn_[version]all.deb	814.61 KB	815.02 KB	420 bytes (0%)

[AlexWayfer]

We have upgrade command for packages, not update, and probably should have self-upgrade instead of self-update. Also, in apt there are update for repositories refresh and upgrade for packages update.

[rafael-vitor]

@AlexWayfer good point, I've put it back as self-update because thats how its still described on the docs: https://yarnpkg.com/lang/en/docs/cli/self-update/ and it was like that until it got removed: 6c41fee.
If its consensus that it should be upgrade I'll happily change it. Maybe the older contributors can help shed some light onto why update was used.

[arcanis]

I believe this feature conflicts in spirit with #6673, which is already in master

[rafael-vitor]

@arcanis I see. Many ppl still refer to #1139 to try and find the way to properly upgrade yarn as the docs still mention it.

Do you think there's room for both commands? Maybe self-upgrade is worth keeping for the clarity of its name. Maybe even as an alias to yarn policies set-version.

Otherwise, we should work on making the community aware of this by linking to it on the current self-update doc and/or removing it eventually.

[bf4]

typo

[AlexWayfer]

installation