(fix): respect tsconfig esModuleInterop flag (#327)

- the previous assumption that ESM users will always import the ESM
  build doesn't actually hold up in practice due to various limitations
  in Node and testing and the prevalence of transpiling in general
  - even if one uses mostly ESM, many tools will require and use
    the CJS build, and this option would break compatibility

- without esModule set, CJS users would be unable to use the default
  export of any library built with tsdx
  - and this was not documented, meaning it would cause unintended
    breaking changes in libraries that use default exports and try to
    adopt tsdx
  - it would also break compatibility with certain tooling

- so, instead of always setting rollup's esModule to false and
  potentially causing unintended consequences, respect the user's
  esModuleInterop config as set in their tsconfig, but default to
  false

src/createRollupConfig.ts

@@ -1,4 +1,9 @@
-import { safeVariableName, safePackageName, external } from './utils';
+import {
+  safeVariableName,
+  safePackageName,
+  external,
+  resolveApp,
+} from './utils';
 import { paths } from './constants';
 import { terser } from 'rollup-plugin-terser';
 import { DEFAULT_EXTENSIONS } from '@babel/core';
@@ -12,6 +17,7 @@ import typescript from 'rollup-plugin-typescript2';
 import { extractErrors } from './errors/extractErrors';
 import { babelPluginTsdx } from './babelPluginTsdx';
 import { TsdxOptions } from './types';
+import * as fs from 'fs-extra';
 
 const errorCodeOpts = {
   errorMapFilePath: paths.appErrorsJson,
@@ -39,6 +45,11 @@ export async function createRollupConfig(opts: TsdxOptions) {
     .filter(Boolean)
     .join('.');
 
+  let tsconfigJSON;
+  try {
+    tsconfigJSON = fs.readJSONSync(resolveApp('tsconfig.json'));
+  } catch (e) {}
+
   return {
     // Tell Rollup the entry point to the package
     input: opts.input,
@@ -58,8 +69,8 @@ export async function createRollupConfig(opts: TsdxOptions) {
       // Do not let Rollup call Object.freeze() on namespace import objects
       // (i.e. import * as namespaceImportObject from...) that are accessed dynamically.
       freeze: false,
-      // Do not let Rollup add a `__esModule: true` property when generating exports for non-ESM formats.
-      esModule: false,
+      // Respect tsconfig esModuleInterop when setting __esModule.
+      esModule: tsconfigJSON ? tsconfigJSON.esModuleInterop : false,
       // Rollup has treeshaking by default, but we can optimize it further...
       treeshake: {
         // We assume reading a property of an object never has side-effects.