Define the programmatic API

[travi]

New feature motivation

As pointed out in #2968, defining exports for this package restricted access to internal files and prevented potentially legitimate use cases. The problem with the situation was that before folks, who were willing to speak up about the change breaking them, were presented with the breakage, we as maintainers did not have insight into the use cases to be able to support them more officially.

We do want to restrict our public API more than we have so that we can properly test for intended behaviors and guard against similar breakages in the future. The goal of defining exports was not to prevent extensibility of semantic-release, so we will be reverting that change. However, we want to keep track of the bits that we've learned to inform how we would more intentionally define the public API as a follow up.

New feature description

expose a programatic API that enables appropriate extensibility beyond the functionality available directly from the cli

New feature implementation

the eventual goal would be to extract the core logic to a package that is consumed by the cli, enabling that package to expose the programmatic API and be consumed more directly. we may choose to expose that API directly from this package at first, but it would likely be removed from this package later, in a breaking change, once the core package is extracted.