feat: use toml-span for deserialization

[baszalmstra]

Fixes #2640

WIP refactor to use toml-span for parsing instead of serde. This gives us much more control over deserialization and the use of spans.

[bollwyvl]

It looks like taplo uses toml-span, which is likely a good sign... though I still don't see a clear path to a "perfect" generated representation of the key structures as JSON schema, (a la #2057 where schemars is mentioned).

Indeed, using more of taplo (and even its semi-icky x-taplo schema stuff) might lead to some opportunities to support (not just) That Editor on #2444.

[Hofer-Julian]

@baszalmstra The integration tests should be fine now with 62af58f