Expose install function

[jlkiri]

In this PR I simply expose the install function without any modifications.

Related issues: #312

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[jlkiri]

I'm not sure if i need to add https://www.npmjs.com/package/@pika/plugin-build-types, because @pika/plugin-ts-standard-pkg is already used and the docs of plugin-build-types say:

Note: This plugin is not needed if you are already writing TypeScript and using the ts-standard-pkg plugin.

[FredKSchott]

+1 for this as a starting point, but with an "unstable" prefix:

export {install as unstable_installCommand} from './commands/install';

Even though we won't document this on snowpack.dev, we still want to make sure that someone who does find it knows that it's not officially supported and could change across even patch versions, until we get further along in the work outlined in #312 and feel comfortable maintaining via semver)

@jlkiri If you can make that change, I'll merge.

[FredKSchott]

reviewed above

[FredKSchott]

LGTM! We actually might need to prioritize a true install JS API sooner rather than later for #475