fix: use type guards for isOk/isFail in Result

[IgnusG]

Hello, just found your library and was trying it out when I noticed the results weren't making use of typescript's type guards so I wanted to propose a small change to support them.

This ensures that:

const a = new OkResult("ok") as Result<string, string>;

if (a.isOk()) {
  const b = a; // Both a and b are narrowed to OkResult;
}

// a is Result outside of the block

Same for isFail

[patrickmichalina]

@IgnusG excellent, thank you!

[patrickmichalina]

🎉 This PR is included in version 6.0.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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