Am I understanding correctly that this basically adds a type system to TOML? Have you considered calling it TypeTOML and making it a superset of TOML? (Maybe this is covered in the readme which I only skimmed.)
It would be cool if I could rename `config.toml` to `config.ttoml` and add types as I need them, similarly to how I can rename `script.js` to `script.ts` for iterative adoption of TypeScript. Although obviously this would require the consuming code to implement the Jesth (TypeTOML?:)) parsing, which would maybe defeat the point of iteratively adopting it (why bother with partial compatibility then?). Perhaps you could make it a _compatible_ superset, with types implemented using TOML comments so that existing TOML parsers can parse an (untyped) structure from a Jesth file by ignoring the comments, while Jesth can parse a typed structure from the same file.