fix: map "es6" to "es2015"

Author: wChenonly

packages/core/tests/syntax.test.ts

@@ -153,13 +153,20 @@ describe('transformSyntaxToBrowserslist', () => {
 
 describe('transformSyntaxToRspackTarget', () => {
   test('esX', () => {
+    const es6 = transformSyntaxToRspackTarget('es6');
     const es2023 = transformSyntaxToRspackTarget('es2023');
     const es2024 = transformSyntaxToRspackTarget('es2024');
     const esnext = transformSyntaxToRspackTarget('esnext');
 
     expect(es2023).toEqual(es2024);
     expect(es2023).toEqual(esnext);
 
+    expect(es6).toMatchInlineSnapshot(`
+      [
+        "es2015",
+      ]
+    `);
+
     expect(es2023).toMatchInlineSnapshot(
       `
       [
@@ -186,5 +193,14 @@ describe('transformSyntaxToRspackTarget', () => {
         "es2022",
       ]
     `);
+
+    expect(
+      transformSyntaxToRspackTarget(['Chrome 123', 'es6']),
+    ).toMatchInlineSnapshot(`
+      [
+        "browserslist:Chrome 123",
+        "es2015",
+      ]
+    `);
   });
 });