feat: prepack script to allow github installs

[rflechtner]

Adds a prepack script to build the package, which is called by yarn 2+ if you install the package from the GitHub source (https://yarnpkg.com/advanced/lifecycle-scripts/#prepack-and-postpack).
After this is merged, you can directly install unreleased versions of this package to your local (yarn 2+) projects, using e.g.

yarn add @kiltprotocol/kilt-extension-api@KILTprotocol/kilt-extension-api

How to test:

• Create a new project using yarn 2 / yarn 3 as package manager
• Install the extension library from this branch, using yarn add @kiltprotocol/kilt-extension-api@KILTprotocol/kilt-extension-api#rf-prepack-script
• Try running the cli, e.g. using yarn createDidConfig --help

Checklist:

• I have verified that the code works
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• I have left the code in a better state
• I have documented the changes (where applicable)
  • Either PR or Ticket to update the Docs
  • Link the PR/Ticket here