Fix Options types by making them writable

[felipecrs]

I tried integrating the API in semantic-release-vsce but found that the Options interfaces are too restrictive.

They don't allow things like:

import { createVSIX, IPackageOptions } from './src/api';

const options: IPackageOptions = {}

if (process.env.SOMETHING === 'false') {
  options.gitTagVersion = true;
}

await createVSIX(options);

Which fails with:

Cannot assign to gitTagVersion because it is a read-only property.

So, after thinking about it, I believe the best solution is to remove the readonly modifier from all properties of Options interfaces. I don't think they serve any purpose, except making the API harder to use in real-world scenarios.

What do you think?