Validate manifests

[itowlson]

Tracking issue for possible validations e.g.

• Are all required fields present?
• Does the source file exist?
• Does the files array contain any patterns that match no files?

[itowlson]

The "Even Better TOML" extension allows validating TOML against a JSON schema, which would achieve what we need, but doesn't yet have a way for an extension to register schemas. I've raised an issue and got a positive response, but they're not in a position for external contributors to send PRs right now; discussing it with the maintainer though.

[radu-matei]

What if we had a Spin command that did some of this work?
It wouldn't have to be as accurate as a "language server" (i.e squiggles on specific lines, although that would be cool), but it could be able to validate a particular manifest.

[itowlson]

If it gave source file locations where appropriate, that would be great, but as you imply, the current architecture wouldn't give us that, because Spin uses serde deserialisation which does not retain position information. Without that, we could produce a problems list, but it wouldn't be very idiomatic. In VS Code the user expects squigglies, and expects to be able to click on a problem in the list and get taken to the location...