enhancement: add custom max pixel length functionality

[hidayahapriliansyah]

While using Thumbhash, I wanted to generate image thumbnails with very small file sizes, ideally less than 1KB. To achieve this, I needed the ability to set a custom maximum pixel length, such as 16 pixels. However, I found that Thumbhash currently enforces a default minimum pixel length of 32 pixels, which limits flexibility.

I made changes to the code to allow users to define their own custom maximum pixel length when generating thumbnails. This enhancement provides more control over the size of the thumbnails, making it possible to optimize them for smaller file sizes when needed. A demo of this updated functionality can be found here: https://thumbhash-generator.vercel.app/