@embroider/macros is not parallelizable

[rwjblue]

Quick example PR over in ember-engines: ember-engines/ember-engines#751

Haven't had much time to dig into this just yet, but it seems like the plugins registered here:

embroider/packages/macros/src/ember-addon-main.ts

Lines 88 to 94 in d21b77e

	registry.add('htmlbars-ast-plugin', {
	name: `@embroider/macros/${index}`,
	plugin,
	baseDir() {
	return join(__dirname, '..');
	},
	});

Need to be including the parallelBabel property (which is used by ember-cli-htmlbars here).

Here is an example from ember-cli-htmlbars own test suite:

    registry.add('htmlbars-ast-plugin', {
      name: 'module-name-inliner',
      baseDir() {
        return __dirname;
      },
      parallelBabel: {
        requireFile: __filename,
        buildUsing: 'buildPlugin',
        params: {},
      },
      plugin(env) {
        let { builders } = env.syntax;

        return {
          name: 'module-name-inliner',

          visitor: {
            PathExpression(node) {
              if (node.original === 'module-name-inliner') {
                // replacing the path with a string literal, like this
                // {{"the-module-name-here"}}
                return builders.string(env.moduleName);
              }
            },
          },
        };
      },
    });