[easy] Add an example script that computes a percentage of a token amount

[davedumto]

Summary

Add a standalone example script under contrib/examples/ that computes a given percentage of a base unit token amount, returning a bigint result.

Requirements

• File lives under contrib/examples/percentage-of-amount/ with a short README.
• Exposes a small percentageOf function used by the script.
• Handles a percentage of zero and of one hundred as edge cases.
• Include a few example inputs and expected outputs in the README.

Constraints

• Fork the repo, then create your branch from the drips branch (NOT main).
• Do ALL your work inside the contrib/ folder. Do not add, edit, rename,
  or delete any file outside contrib/.
• Open your PR against the drips branch. PRs targeting main are
  auto-closed. PRs targeting drips that touch files outside contrib/ are
  also auto-closed.
• Save your work under contrib/examples/ named after this issue (for example,
  contrib/examples/issue-42-name/).

Acceptance Criteria

• The change is entirely inside contrib/.
• The PR base branch is drips.
• Reviewed and approved by a maintainer before merge to drips.
• The maintainer will merge drips into main separately.

[olayiwola-web]

@olayiwola-web has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer, I’ve reviewed the requirements and can take this on. I will keep the example self-contained under contrib/examples/percentage-of-amount/, implement percentageOf using bigint-safe arithmetic, cover the 0% and 100% cases, and document a few concrete inputs and expected results. I will also work from drips and keep the PR strictly within contrib/.
Thanks

ℹ️ Repo Maintainers: To accept this application, review their application or assign @olayiwola-web to this issue.