Replace `Nett` TOML library

[JamieMagee]

Currently we use the Nett library for parsing TOML files. However, it appears that it has been deprecated. The package on NuGet.org hasn't been updated in over 2 years¹ and the GitHub repository has been archived².

It looks like the most popular alternative library is Tomlyn³

Footnotes

1. https://www.nuget.org/packages/Nett ↩

2. https://github.com/paiden/Nett ↩

3. https://www.nuget.org/packages/Tomlyn ↩

[JamieMagee]

Link to the Tomlyn documentation: https://github.com/xoofx/Tomlyn/blob/main/doc/readme.md