docs(porting addons to v2): change the recommended package manager to pnpm

[BlueCutOfficial]

This PR is a proposal to change the recommended package manager to pnpm in the Porting Addons to V2 guide.

My understanding is that pnpm is better than yarn to work with Embroider in general, so it may be preferable to recommend pnpm in this document rather than making it more generic about the package manager part.

[NullVoxPopuli]

Thanks! Looks great!

[sandstrom]

I'm a bit late to the party here, but I would recommend this approach:

I think Ember should pick the package manager that most people are familiar with as its default. Currently, that is NPM. Yes, there are better alternatives (which one is better depend on the project/team, for some the "best manager" is Yarn, for others it is pnpm).

But NPM, with all its flaws, is still the de-facto standard NPM package manager. To keep friction down, I think that should be the Embroider default.

This doesn't preclude us from making it easy to use other package managers, or even offer it as an command line option --package-manager=pnpm.

Also, for everyone discussing this. Keep in mind that this choice shouldn't be optimized on what you (all tech savvy people with tons of JS/front-end experience) knows is "the best choice". Instead, it should be optimized on what makes Ember easy to get started with for the average developer. For all of you, it'll be a breeze to pick another package manager.

-- emberjs/rfcs#820 (comment)

[mansona]

thanks @sandstrom for the link. That concern isn't entirely relevant for this PR because this is just for the internal documentation that is being used to guide people through the process of upgrading to a v2 addon. When we open this process to a RFC (which will happen once we are ready to make it the default) then we can start sweating the details of our choices 👍