Allow post install script

[robin-drexler]

Snapit does a checkout of the code instead of running in the existing action context, which makes it hard to make changes to files before snapit runs.

build_script exists, but it runs after snapit has already ran changeset version which might be too late in certain cases.

This PR introduces a post_install_script optional input param that allows to pass a script that runs right after dependencies have been installed.

In our specific instance we want to run changeset pre exit to allow snaphots for a package that is currently in pre mode.
More on this: https://changesets-docs.vercel.app/en/prereleases

You can't version snapshot packages when pre mode is on.
We do have pre mode on for the next RC in https://github.com/Shopify/ui-extensions/ because it allows us to release versions like 2025-10.rc-1 and add ~2025-10.rc-0 to extension templates as a workaround to not being able to put a tag there in npm: npm/cli#8252

We might want to have snapit do changeset pre exit by itself at some point, but I think it's too early to add this.
We're still in the process of figuring out how pre mode works exactly and I think having a more generic post install hook is a sensible addition regardless of our current use case.

🎩

Tested successfully in the ui-extensions repo:
Shopify/ui-extensions#2901 (comment)
https://github.com/Shopify/ui-extensions/actions/runs/15144950812

[robin-drexler]

prettier did this

[rxbchen]

No problems with this!

[vividviolet]

👍 This is a good addition to snapit

[alex-page]

@robin-drexler why not just include it in your package as a postinstall script? Unsure why snapit needs to have this as a feature. Is there a time when you don't want this postinstall script to run?

Seems like it's just adding an extra place for functions to run that is unnecessary.

[robin-drexler]

@robin-drexler why not just include it in your package as a postinstall script? Unsure why snapit needs to have this as a feature. Is there a time when you don't want this postinstall script to run?

Seems like it's just adding an extra place for functions to run that is unnecessary.

We don't want to run this every time after dependencies are installed. Only when running in the snapit gh action because you can't create a snapshot version if you're in pre mode so we're temporarily disabling it in the snapit action.

[alex-page]

I want to be careful about adding multiple lifecycle hooks and focus on keeping snapit extremely simple. This makes sense though, thanks @robin-drexler