Investigate if square size can be added as a parameter to `TreeConstructorFn`

## Prerequisite

1. https://github.com/celestiaorg/rsmt2d/issues/248

## Context

https://github.com/celestiaorg/rsmt2d/blob/76d744060bc715801cd9e01119bfb1dea00dc488/tree.go#L9-L11 lacks a parameter for square size so multiple tree constructor functions must be registered (one per square size).

## Proposal

Explore if it's possible to add a square size param to the tree constructor function.

Start with prerequisite issue #248, then read tree.go lines 9–11 where TreeConstructorFn is defined. Investigate how square size is currently handled and why multiple tree constructor functions are registered. Done means documenting whether a square-size parameter is feasible and identifying the resulting API changes.

Written by the indexing model from the issue text.