Post #3178711

Replaced our usage of the `toml` crate with `toml-spanner` and am fairly happy with the result. The error messages coming out of `toml` (really out of `serde`) were nonsensical for users and this work was primarily motivated by an upcoming feature where we will have certain strings in the TOML file that are snippets of a DSL. With `toml-spanner`, we can now directly parse and statically analyze those snippets and offset any diagnostics based on the TOML string's span within the file, leading to less frustrated users.