Add missing index_of() fn

[0xjei]

Description

This PR aims to add an index_of() fn to IMT impl to mirror the TS implementation. The function takes a leaf and returns an Option<usize> that evaluate to Some(idx) where idx is the index of the leaf or None when the leaf does not exists in the IMT.

Checklist

• I have read and understand the contributor guidelines and code of conduct.
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have run yarn style without getting any errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Important

We do not accept minor grammatical fixes (e.g., correcting typos, rewording sentences) unless they significantly improve clarity in technical documentation. These contributions, while appreciated, are not a priority for merging. If there is a grammatical mistake, please feel free to message the team.