Initial work for versionsuffix and external-id

[alexcrichton]

This is intended to at least start laying groundwork and implementation for two features in the component model that this repository does not yet implement:

• (versionsuffix "...") for components -- introduced in Interface version canonicalization WebAssembly/component-model#536 this is intended to pave a path forward to canonicalizing versions in component imports/exports in the future. For example where today you'd import a:b/c@1.0.0 tomorrow you'd import a:b/c@1. This is not yet fully integrated into wit-component, however, and further work will be necessary for bindings generators as well to take advantage of this. It's all encoded in the same place as external-id, though, so I figured I'd at least start to lay the groundwork.

• (external-id "...") for components -- being introduced in Add external-id attribute WebAssembly/component-model#672 this is intended to attach arbitrary metadata to imports/exports in a component. This is surfaced in WIT as an @external-id("...") attribute. This should be hooked up throughout this implementation to WIT and such such that the implementation is intended to be complete. There's probably something I forgot, but all the major pieces should be there.

The versionsuffix parsing/validation is gated by a new cm-canon-names feature, and the external-id parsing/validation is gated behind the preexisting cm-implements feature to match the specification's classification.

Closes #2551

[ricochet]

Heads up that gating external-id to instances is different from the spec PR. IMO gating on instances is the right call so we should funnel that back to the spec. AKA right now this PR has external_id only on WorldItem::Interface, not on Function/TypeDef.

Right now the Explainer's motivating example puts external-id on a bare func import:

(component $ESM
  (import "slugify"
    (external-id "https://esm.unpkg.com/slugify@1.6.6")
    (func (param "text" string) (result string)))
)

[fitzgen]

LGTM