[WIP/do not merge] Replace anyhow with typed errors in wit-parser

[PhoebeSzmucer]

Motivation

Today, getting a file path and line number out of a parse error requires parsing the human-readable error string that anyhow produces. This is fragile: it breaks silently when error message formatting changes, and it's simply the wrong abstraction.

I want to use wit-parser in an LSP for WIT/WAC, but this change would be good for any tool that embeds wit-parser and wants to do something useful with errors.

I Know This Is Large

I'm aware this PR is bigger than ideal. The core reason it's large is that error type changes propagate through the entire call stack - there's no natural seam to split at. If this is something that we'd like to merge in some form, I can try splitting up this PR into smaller chunks - I wanted to open this draft first to start a conversation.

Global vs local spans

Currently I'm returning global Span objects as part of WitError, because existing code uses global spans almost exclusively. This makes things annoying for the consumer - they have to use the SourceMap object to map to file-local offsets. I'm considering changing the exposed spans to be resolved local spans.