RFC: Upstream Forgejo/Gitea support

[openscript]

Hi! We are using Changesets successfully across Codeberg and self-hosted Forgejo instances. The only component requiring adjustments for Gitea/Forgejo API compatibility is this action.

I have already implemented and tested these changes in a fork:
https://github.com/openscript/changesets-action

Key points:

• Fully backward compatible with GitHub (no changes to existing workflows).
• Enables Changesets for Forgejo, Gitea, and Codeberg users.

Would you be open to a PR merging this upstream? Happy to open one and adapt the implementation to your preferred patterns.

[Andarist]

It would be good if u’d post a list of changes that you had to introduce to add that support

[openscript]

Changes I needed to introduce:

• Platform detection: https://github.com/openscript/changesets-action/blob/1967829df87d04313e295397cc26365b1c2c1eba/src/run.ts#L28-L34
• List pull requests (manual filtering by head and base, because not supported via REST API): https://github.com/openscript/changesets-action/blob/1967829df87d04313e295397cc26365b1c2c1eba/src/run.ts#L62-L79
• Update pull request via REST (not supported via Graphql): https://github.com/openscript/changesets-action/blob/1967829df87d04313e295397cc26365b1c2c1eba/src/run.ts#L512-L523