Let plugins supply their own static import() loader

[DmitrySharabin]

Problem

Plugins are loaded by URL, so they sit outside the static module graph and import-map/bundler tooling can't trace their dependencies.

Fix

load() honors an optional load thunk on the registry entry (a static import()), falling back to URL loading for plugins known only by base.

Related PRs

Coordinated change across the repos:

• Trace plugin dependencies via static loaders plugins#3 — static plugin loaders (use this load field)
• Drop the @inspirejs/core/util workaround inspire.js#176 — drops the now-redundant @inspirejs/core/util workaround
• Load markdown-it as a local dependency instead of from a CDN plugins#4 — markdown-it as a local dep (consumes the above)

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