feat: Add customizable conflict marker labels & Linting

[shiueo]

Summary

This PR adds the ability to customize conflict marker labels in merge operations, allowing users to specify custom labels instead of the default "ours", "theirs", and "original".

Changes

• Added ours_label, theirs_label, and original_label fields to MergeOptions
• Added methods to set individual labels: set_ours_label(), set_theirs_label(), set_original_label()
• Added convenience method set_conflict_labels() to set all labels at once
• Updated output_result() and output_result_bytes() functions to use custom labels

Example Usage

let result = MergeOptions::new()
    .set_ours_label("main")
    .set_theirs_label("feature-branch")
    .set_original_label("base")
    .merge(ancestor, ours, theirs);

let result = MergeOptions::new()
    .set_conflict_labels("HEAD", "incoming", "common-ancestor")
    .merge(ancestor, ours, theirs);

Backward Compatibility

This change is fully backward compatible. Existing code will continue to work with the default labels ("ours", "theirs", "original").