Add --yarn option to use yarn instead of npm

[glensc]

Fixes #289

use yarnInstall if --yarn option is used:

• https://yeoman.io/authoring/dependencies.html

[glensc]

I don't know how to test this without code being published in npm registry.

[oklas]

It is possible to install package for testing. Package may be installed from local folder or from tarball, both yarn and npm.

[glensc]

Probably more changes needed, as this did not have any effect at all

[oklas]

Perhaps yes

[oklas]

Moreover, I do not see similar testing for npm either. But I noticed that the readme template generator should also be changed here

[glensc]

PR for readme sub-generator:

• feat: add yarn/npm install to readme on yarn condition #294

let's get that resolved and merged first.

[glensc]

Rebased after #294 merge

I don't use this anymore, so either merge or close.

[JoshuaKGoldberg]

👋 Thanks for the merge->rebase! Coming over from yeoman/yeoman#1779, we're still ramping up on PR infrastructure and reviews. Lots of things are broken and need to be overhauled. But we're hopeful we'll get to this Soon ™️!

[mshima]

npmInstall/yarnInstall apis has been removed.
Install task is executed automatically by the environment if package.json change was detected (if the file was committed).

• Client manager can be passed by --node-package-manager option otherwise it is detected by lock file existence and falls back to npm.
• A generator can force a pm using constructor's features param.

Just realized that corepack integration can be added. https://nodejs.org/api/packages.html#packagemanager.
This will simplify the package manager selection.