[config plugins] extend base mods

[EvanBacon]

Why

Follow up: #3839 (comment)

How

• Added static mod property isIntrospective which will prevent a base mod from being removed during introspection.
• Combined introspection providers with regular providers for a more unified interface, and as a way to test the public interface for introspection providers.
• Exposed BaseMods.provider helper and BaseMods.withGeneratedBaseMods so users could create their own base mods locally and add them to the compiler sequence.

Test Plan

• Existing tests
• Locally using:

import {
  ConfigPlugin,
  IOSConfig,
  Mod,
  withMod,
  BaseMods,
} from "../../cli/packages/config-plugins/build/index";
import fs from "fs";

export function withCustomBaseMods(config) {
  return BaseMods.withGeneratedBaseMods<"appDelegateHeader">(config, {
    platform: "ios",
    skipEmptyMod: false,
    providers: {
      // Append a custom rule to supply AppDelegate header data to mods on `mods.ios.appDelegateHeader`
      appDelegateHeader:
        BaseMods.provider<IOSConfig.Paths.AppDelegateProjectFile>({
          isIntrospective: true,
          getFilePath({ modRequest: { projectRoot } }) {
            return IOSConfig.Paths.getAppDelegateHeaderFilePath(projectRoot);
          },
          async read(filePath) {
            return IOSConfig.Paths.getFileInfo(filePath);
          },
          async write(filePath, { modResults: { contents } }) {
            await fs.promises.writeFile(filePath, contents);
          },
        }),
    },
  });
}

/**
 * Provides the AppDelegate header file for modification.
 *
 * @param config
 * @param action
 */
export const withAppDelegateHeader: ConfigPlugin<
  Mod<IOSConfig.Paths.AppDelegateProjectFile>
> = (config, action) => {
  return withMod(config, {
    platform: "ios",
    mod: "appDelegateHeader",
    action,
  });
};

export const withCustomHeaderInterface = (config) => {
  return withAppDelegateHeader(config, (config) => {
    console.log("modify header:", config.modResults);
    return config;
  });
};

app.config.js

// Required for external files using TS
require("ts-node/register");

import {
  withCustomBaseMods,
  withCustomHeaderInterface,
} from "./withAppDelegateHeaderBaseMod.plugin.ts";

export default ({ config }) => {
  if (!config.plugins) config.plugins = [];
  config.plugins.push(
    withCustomHeaderInterface,
    // Base mods MUST be last
    withCustomBaseMods
  );
  return config;
};