Add documentation for the abi keyword for LSP

[JoshuaBatty]

Description

As the title says.

There's a corresponding PR in the vscode plugin for correct syntax highlighting when hovering over the abi keyword here FuelLabs/sway-vscode-plugin#193

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #7148 will not alter performance

_{Comparing josh/abi_highlighting (503726e) with master (556b95a)}

Summary

✅ 22 untouched benchmarks