fix code snippet for old diagnostics

[xunilrj]

Description

This PR solves a small bug when calculating the code snippets of some diagnostics.

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.

[alfiedotwtf]

Just thinking, maybe a few simple tests that do the happy path and also bounds checks might be worth it so we don't introduce unwanted behaviour?

[xunilrj]

I have changed the algorithm to something easier to understand, more performant, given that we iterate the string only once, and I have also written more tests.