feat(babel-jest): support .mts, .cts coverage

Istanbul only collects coverage from files listed in 'extension'. It has a [default list](https://github.com/istanbuljs/schema/blob/master/default-extension.js) of
`.js, .cjs, .mjs, .ts, .tsx, .jsx`. Any other files are ignored. As babel-jest doesn't provide the extension, or a way to provide it yourself, there is currently
no way to collect coverage from .mts, .cts files. I've [proposed adding them to the default Istanbul list](istanbuljs/schema#22). But in the
same fashion that Jest removes all exclusions, it could also provide the extension of the file being transformed to allow Istanbul to collect it's coverage.

I can't think of a situation where you would want a file transformed by babel, but not to be instrumented that wouldn't already be handled by jests coverage include/exclude config.

Author: domarmstrong

packages/babel-jest/src/__tests__/index.ts

@@ -18,6 +18,7 @@ import babelJest, {createTransformer} from '../index';
 // written in ESM and we don't support require(esm) yet.
 import Module from 'node:module';
 import {pathToFileURL} from 'node:url';
+
 const createOriginalNodeRequire = Object.getPrototypeOf(Module).createRequire;
 const originalNodeRequire = createOriginalNodeRequire(
   pathToFileURL(__filename),
@@ -247,4 +248,32 @@ function defineTests({
       expect.objectContaining({presets: []}),
     );
   });
+
+  describe('instrument', () => {
+    test('adds the babel istanbul plugin', async () => {
+      defaultBabelJestTransformer.process(sourceString, 'dummy_path.mjs', {
+        cacheFS: new Map<string, string>(),
+        config: makeProjectConfig(),
+        configString: JSON.stringify(makeProjectConfig()),
+        instrument: true,
+        transformerConfig: {},
+      } as TransformOptions<BabelTransformOptions>);
+
+      expect(mockedBabel.transformSync).toHaveBeenCalledTimes(1);
+      expect(mockedBabel.transformSync).toHaveBeenCalledWith(
+        sourceString,
+        expect.any(Object),
+      );
+      expect(mockedBabel.transformSync.mock.calls[0][1]?.plugins).toEqual([
+        [
+          require.resolve('babel-plugin-istanbul'),
+          {
+            cwd: '/test_root_dir/',
+            exclude: [],
+            extension: ['.mjs'],
+          },
+        ],
+      ]);
+    });
+  });
 }

packages/babel-jest/src/index.ts

@@ -51,6 +51,7 @@ function assertLoadedBabelConfig(
 }
 
 function addIstanbulInstrumentation(
+  filename: string,
   babelOptions: BabelTransformOptions,
   transformOptions: JestTransformOptions,
 ): BabelTransformOptions {
@@ -66,6 +67,7 @@ function addIstanbulInstrumentation(
           // files outside `cwd` will not be instrumented
           cwd: transformOptions.config.cwd,
           exclude: [],
+          extension: [path.extname(filename)],
         },
       ],
     ];
@@ -142,7 +144,7 @@ function loadBabelOptions(
 ): BabelTransformOptions {
   const {options} = loadBabelConfig(cwd, filename, transformOptions);
 
-  return addIstanbulInstrumentation(options, jestTransformOptions);
+  return addIstanbulInstrumentation(filename, options, jestTransformOptions);
 }
 
 async function loadBabelOptionsAsync(
@@ -153,7 +155,7 @@ async function loadBabelOptionsAsync(
 ): Promise<BabelTransformOptions> {
   const {options} = await loadBabelConfigAsync(cwd, filename, transformOptions);
 
-  return addIstanbulInstrumentation(options, jestTransformOptions);
+  return addIstanbulInstrumentation(filename, options, jestTransformOptions);
 }
 
 export const createTransformer: TransformerCreator<