Add megahit module (short-read assembler)

### Description of feature

## Description
Implement a module for [MEGAHIT](https://github.com/voutcn/megahit), a fast and memory-efficient assembler designed for assembling Illumina short reads, especially in metagenomic contexts.

## Goal

- Create an nf-core module that runs MEGAHIT
- Add test data and example config
- Include documentation