DEV/ENH: Switch to using tomlkit

[NickleDave]

We rely on toml but I don't think it's actively maintained.
It also does not preserve formatting in some cases, e.g. when we add dataset_path at the end of the prep step and re-save the TOML file, it will convert in-line tables into their own separate tables

For example,

[PREDICT]
transform_params = {window_size = 44}

becomes

[PREDICT.transform_params]
window_size = 44

• Switch to using tomlkit, that is still maintained
• do this before change config file format to leverage toml tables #345