Fix yarn init printing undefined values

[Zach10za]

What's the problem this PR addresses?
This PR fixes #624

To summarize the issue, when running yarn init in a new project, the unused fields are all listed with the value undefined. This is because in Manifest.ts exportTo method, fields in the data object are being set as undefined if not used.

Here is an example:

> yarn init
{
  name: 'example-project',
  bin: undefined,
  dependencies: undefined,
  optionalDependencies: undefined,
  devDependencies: undefined,
  peerDependencies: undefined,
  dependenciesMeta: undefined,
  peerDependenciesMeta: undefined,
  resolutions: undefined,
  files: undefined
}

How did you fix it?

Now in the Manifest.ts exportTo function, instead of setting unused fields as undefined they get deleted from the data object. The new output looks like this:

> yarn init
{ name: 'example-project' }

In order to get proper formatting, the compact option is now set to false in init.ts executeRegular. The final output looks like this:

> yarn init
{
  name: 'example-project'
}

Other solutions

This solution touches a lot of code. An easier fix could be adding something like this to the bottom of the exportTo function:

// Remove all keys with 'undefined' value
for (const key in data)
  if (data[key] === undefined)
    delete data[key];

I opted to go the other route because it seemed more explicit and I think it's better to handle the individual cases closer to the actual logic.

[arcanis]

Looks perfect, thanks! 👌