Issue number: resolves #30982, resolves #30686, resolves #30730

---------

## What is the current behavior?

Core's package.json has no `exports` field. Webpack and Vite handle that
fine, but Node's strict ESM resolver ignores nested package.json files
and refuses directory imports. So when the built `@ionic/angular` bundle
does `import { defineCustomElements } from '@ionic/core/loader'` or
`import { componentOnReady } from '@ionic/core/components'`, anything
running under Node ESM (Angular 21's default Vitest builder, raw Node,
etc.) throws `ERR_UNSUPPORTED_DIR_IMPORT`. All three resolved issues
have the same root problem

## What is the new behavior?

Core's package.json now declares an `exports` map that covers every
directory in `files` with explicit `index.js` resolution where directory
imports happen, and wildcards for the deep paths

## Does this introduce a breaking change?

- [x] Yes
- [ ] No

When you add `exports` to a package.json, it becomes an allowlist. This
means that every undocumented import path will break unless you're using
`moduleResolution: node`

Related docs PR: ionic-team/ionic-docs#4523

## Other information

Apps already on `moduleResolution: "bundler"` (Angular 17+ default, Vite
default) pick up the fix automatically. Apps on classic
`moduleResolution: "node"` keep working through the legacy fields and
are unaffected.

Current dev build:
```
8.8.9-dev.11780086312.13aac420
```