Add inferring of development options from webpack loader mode

Closes GH-2198.
Closes GH-2201.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

packages/loader/lib/index.js

@@ -35,7 +35,10 @@ const cache = new WeakMap()
 export function loader(value, callback) {
   /** @type {Defaults} */
   const defaults = this.sourceMap ? {SourceMapGenerator} : {}
-  const options = /** @type {CompileOptions} */ (this.getOptions())
+  const options = {
+    development: this.mode === 'development',
+    .../** @type {CompileOptions} */ (this.getOptions())
+  }
   const config = {...defaults, ...options}
   const hash = getOptionsHash(options)
   // Some loaders set `undefined` (see `TypeStrong/ts-loader`).

packages/loader/test/index.test.js

@@ -95,6 +95,13 @@ webpack.mdx:1:22: Unexpected end of file in expression, expected a corresponding
     'should compile (react)'
   )
 
+  const reactOutput = await fs.readFile(new URL('react.cjs', base), 'utf8')
+  assert.not.match(
+    reactOutput,
+    /react_jsx_dev_runtime__WEBPACK_IMPORTED_MODULE_\d+__\.jsxDEV/,
+    'should infer the development option from webpack’s production mode'
+  )
+
   await fs.unlink(new URL('react.cjs', base))
 
   // Preact and source maps
@@ -138,8 +145,15 @@ webpack.mdx:1:22: Unexpected end of file in expression, expected a corresponding
     'should compile (preact)'
   )
 
+  const preactOutput = await fs.readFile(new URL('preact.cjs', base), 'utf8')
+  assert.match(
+    preactOutput,
+    /preact_jsx_dev_runtime__WEBPACK_IMPORTED_MODULE_\d+__\.jsxDEV/,
+    'should infer the development option from webpack’s development mode'
+  )
+
   assert.match(
-    String(await fs.readFile(new URL('preact.cjs', base))),
+    preactOutput,
     /\/\/# sourceMappingURL/,
     'should add a source map if requested'
   )