Declare subpath exports for bundled plugins, Eleventy-maintained data, etc.

[noelforte]

Is your feature request related to a problem? Please describe.

Upfront note: I'd be willing to submit a PR to help out with this.

Commit c505822 inadvertantly introduced some breaking changes to subpath exports when Eleventy is consumed as a dependency. By adding the exports key to package.json with only 1 conditional import (import/requre keys), any attempt to import a subpath throws an error.

Describe the solution you'd like

This is more specific to @11ty/eleventy/src/Engines/Util/ContextAugmenter.js (#3355 (comment)) but is applicable to any exports in Eleventy that could be consumed by outside packages, including (but not limited to):

• Plugins bundled with Eleventy
• .d.ts declaration files (#3097, #3296)
• Eleventy-maintained global data and methods like augmentKeys

Syntax I'm thinking of could be something like:

"exports": {
  ".": {
    "import": "./src/Eleventy.js",
    "require": "./src/EleventyCommonJs.cjs",
  },
  "./user-config": {
    "types": "./types/UserConfig.d.ts"
  },
  "./plugins/*": "./src/Plugins/*",
}

The above declarations can be consumed as so:

import type UserConfig from '@11ty/eleventy/user-config';
import Eleventy from '@11ty/eleventy';
import HtmlBasePlugin from '@11ty/eleventy/plugins/HtmlBasePlugin.js';
// ...

Or via an aggregator:

"exports": {
  "./internal-plugins/*": "./src/Plugins/index.js",
}

import {
  HtmlBasePlugin,
  RenderPlugin,
  InputPathToUrlPlugin,
} from '@11ty/eleventy/internal-plugins';

Describe alternatives you've considered

no response

Additional context

Subpath exports are supported by Node 18 which is the required minimum for 3.0. Additionally, when an exports field is present the main field isn't looked at by Node.js except in the case of require() statements, but with a exports[path].require field it's redundant to have for Node 18+.