Feature: extensible plugins

[jwr1]

This proposal would allow for plugins to expose a custom API for other plugins to use. It better follows Rollup's already existing plugin communication design compared to #171 so we can follow a more general implementation of Rollup's api. It also adds the ability to create a new instance of plugins (in case one does not already exist), suggested in #173.

Proof of Concept

Define API:

interface API {
  presets: Record<string, unknown>
}

export const unplugin = createUnplugin<Options, API>((options) => {
  return {
    name: 'unplugin-auto-import',

    api: {
      //...methods and properties exposed for other plugins
      presets: {
        vue: '...',
        react: '...',
      }
    },

    buildStart() {
      const presets = this.api.presets; // Access own API object through context
    },
  };
});

Use in another plugin:

interface Plugins {
  UnpluginAutoImport: import('unplugin-auto-import')
}

export const unplugin = createUnplugin<Options, API, Plugins>((options) => {
  return {
    name: 'unplugin-auto-import-presets',

    plugins: [
      {
        use: 'unplugin-auto-import',
        orderAfter: true
      },
    ],

    buildStart() {
      this.plugins.UnpluginAutoImport.presets.newPreset = '...';
    }
  };
});

Plugin import interface:

interface PluginImport {
  /** module name or UnpluginInstance */
  use: string | UnpluginInstance;
  /** override context key that the imported plugin's api is injected to */
  as?: string;
  /** default creates new instance if one does not already exist, otherwise, creation follows boolean value */
  create?: boolean;
  /** if plugin is created, the options to pass to plugin */
  options?: unknown;
  /** if plugin is created, execute hooks after  */
  orderAfter?: boolean;
}

[jwr1]

I'll close this for now because #176 was merged and provides similar functionality.