Create to fixed function

[Davidjustice28]

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Issue

• #541

Changes

• Created a new to_fixed_string function in strings module
• Added unit test covering multiple edge cases

Notes

Functionality works, but I feel like my solution is a lot of code. I was fighting circle dependency issues and had to use probably too many externals. I am looking forward to this review. First time contributing to open source and only week using gleam so bare with me.