Write function to change a line of code

# What?

Write a wrapper around the [`string_modify_line`](https://github.com/democritus-project/d8s-strings/blob/cd74ba0b7fa0fcb27d3da71a7a1bc95f98f0608d/d8s_strings/strings.py#L22) which takes a string or ast node and modifies a given line of it with a given operator.

# Why?

This is very useful when modifying code.
