chore: add full abi coder tests

[petertonysmith94]

• Relates to Abi - Refactor / Coder #3031

Summary

• We have added a fully featured contract (abi-contract) that takes fixed inputs and returns an expected value.
  • As @nedsalk mentioned, Sway performs some precompilation.
• We have implemented tests for each of the contract functions, and we are currently using our existing encoding package, @fuel-ts/abi-coder.
  • This setup will enable us to swap out the Interface for our newly refactored package with complete confidence and no risk of regression.
• I have introduced a new custom Vitest matcher, toEqualBn, to simplify the validation of returned BN values.

Checklist

• All changes are covered by tests (or not applicable)
• All changes are documented (or not applicable)
• I reviewed the entire PR myself (preferably, on GH UI)
• I described all Breaking Changes (or there's none)

Addressed the requested changes

[github-actions]

Coverage Report:

Lines	Branches	Functions	Statements
64.13%(+0%)	69.71%(+0%)	72.66%(+0%)	64.57%(+0%)

Changed Files:

Coverage values did not change👌.