fix(test): support importing from ES modules in spec tests (#4136)

This adds support for importing from ES modules (w/ the `.mjs`
extension) in spec tests by ensuring that

1. files with such an extension are passed to the jest preprocessor
   (which then runs them through typescript)
2. our typescript helper for string-to-strong transpilation will emit
   `.mjs` files instead of just dropping them on the floor

Together these changes will allow running specs which import `.mjs`
files.

See #3251 for the original issue report

Author: alicewriteswrongs

scripts/bundles/helpers/jest/jest-preset.js

@@ -31,7 +31,7 @@ module.exports = {
   testPathIgnorePatterns: ['/.cache', '/.stencil', '/.vscode', '/dist', '/node_modules', '/www'],
   testRegex: '(/__tests__/.*|\\.?(test|spec))\\.' + moduleExtensionRegexp + '$',
   transform: {
-    '^.+\\.(ts|tsx|jsx|css)$': path.join(testingDir, 'jest-preprocessor.js'),
+    '^.+\\.(ts|tsx|jsx|css|mjs)$': path.join(testingDir, 'jest-preprocessor.js'),
   },
   watchPathIgnorePatterns: ['^.+\\.d\\.ts$'],
 };

src/compiler/transpile/transpile-module.ts

@@ -88,9 +88,11 @@ export const transpileModule = (
       return normalizePath(fileName) === normalizePath(sourceFilePath) ? sourceFile : undefined;
     },
     writeFile: (name, text) => {
-      if (name.endsWith('.js.map')) {
+      if (name.endsWith('.js.map') || name.endsWith('.mjs.map')) {
         results.map = text;
-      } else if (name.endsWith('.js')) {
+      } else if (name.endsWith('.js') || name.endsWith('.mjs')) {
+        // if the source file is an ES module w/ `.mjs` extension then
+        // TypeScript will output a `.mjs` file
         results.code = text;
       }
     },