[ENHANCEMENT] feat: add skip-install alias to skip-npm

[IgnaceMaes]

The --skip-npm flag seems rather confusing now that three package managers are supported.

e.g. a valid way to generate an Ember app with pnpm today is:

ember new example-app --pnpm --skip-npm

With this alias it could be written as follows:

ember new example-app --pnpm --skip-install

I don't have a lot of context about the impact of this change, so please let me know if it isn't as simple as just adding an alias 😄

[NullVoxPopuli]

lgtm -- do we have any docs/guides pages that need updated as well?

[IgnaceMaes]

There doesn't appear to be any references in the guides.

In the cli docs there are three references for which I opened a PR here: ember-learn/cli-guides#308

[kategengler]

Thank you!