[feat] Better errors when deserializing capabilities

[jsimonrichard]

Describe the problem

I missed in the docs (if it's there at all) that plugin names can't have underscores in them. I would have figured this out pretty quickly had the error message for serializing capabilities/default.json been more specific. However, all I got from the build script was

thread 'main' panicked at trellis/src-tauri/build.rs:14:6:
  failed to run tauri-build: failed to parse JSON: data did not match any variant of untagged enum CapabilityFile

Describe the solution you'd like

Normally, serde would give a more specific error, but it can't because it doesn't know which variant of the untagged enum to compare the file to. Based on serde-rs/serde#2157 (comment), this could be mitigated by using https://docs.rs/serde-untagged/latest/serde_untagged/index.html.

I spun up a quick-and-dirty implementation for only /core/tauri-utils/src/acl/capability.rs. I'll open a PR for it in a moment, but it obviously needs work. Before it's merged (if it is going in the right direction), we would need to find all similar instances present in other files.

The result is an error like this:

  thread 'main' panicked at build.rs:11:6:
  failed to run tauri-build: failed to parse JSON: identifiers can only include lowercase ASCII, hyphens which are not leading or trailing, and a single colon if using a prefix at line 16 column 23

It's not perfect; it would also be nice to wrap this error with some information about which file is throwing the error.

Also, it would be nice if the "no underscore" requirement for plugin names is stated in the docs. I think it might go nicely here: https://v2.tauri.app/develop/plugins/, but I don't have time to make another PR.

Alternatives considered

Besides writing our own deserializer without serde_untagged, I don't know of any other options.

Additional context

No response