Support adding scoped packages via snowpack cli

[ReidBraswell]

I tried adding a package via snowpack add; however, this package is under a npm scope: @reach/dialog. I looked at the add command in the snowpack/src/commands directory, and it appears the addCommand function splits the command line argument for the package name based on @ to get the name and the version.

https://github.com/snowpackjs/snowpack/blob/41c29f25e53695ec84c07386af7eb140d0b22a8e/snowpack/src/commands/add-rm.ts#L20

This works great for non-scoped packages; however, for packages published under a npm scope, i.e. @reach/dialog this will fail.

[rizrmd]

this also happens with @fluentui/react, etc.... I tried to change just that line into:

 let [pkgName, pkgSemver] = addValue.indexOf('@') <= 0 ? [addValue, ''] : addValue.split('@');

But the error become even stranger, and I give up.

[rizrmd]

Here is my patch-package patch file, just in case someone interested:

snowpack+3.0.11.patch.bak.zip

p.s. remove .bak from the file name.