feat: ensurePackageInstalled utility

[dnldsht]

Proposal for #53

I've done something similar as shown in the references by @antfu
I've called addDependency. Should I make it more complex?

[antfu]

I think for ensurePackageInstalled we should minimalize the overhead to shortcircuit the CLI executing when the package is installed.

[dnldsht]

@antfu I'm willing to shortcircuit the CLI executing when the package is installed, but I'm not sure how to proceed, these are my ideas:

• use yarn list, pnpm list or npm list
• check package.json

Should it also check the version of the package e.g. ensurePackageInstalled('nuxt@^3.0.0') must trigger addDependency if a lower version of nuxt is installed.

[antfu]

As you see in Vitest, we checked it using local-pkg. According to @pi0, I think here we plan to also port the functionality in local-pkg to be self-included.

[pi0]

Thanks for the PR dear @dnldsht. A different implementation of ensureDependencyInstalled is merged via (rather big) #75 PR.

I would appreciate your review and feedback to improve that one ❤️