[js][student_grades][extract_class] Add base

Author: rgomezcasas

examples/js/js-student_grades-03_extract_class/.gitignore

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+node_modules/
+.tmp/

examples/js/js-student_grades-03_extract_class/babel.config.json

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+{
+    "presets": ["@babel/preset-env"],
+    "plugins": ["@babel/plugin-proposal-class-properties"]
+}

examples/js/js-student_grades-03_extract_class/jest.config.js

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+module.exports = {
+    testEnvironment: 'node',
+    verbose: true,
+    cacheDirectory: './.tmp/jestCache',
+    transform: {'^.+\\.js$': 'babel-jest'},
+    testMatch:[ "**/tests/**/*.[jt]s?(x)", "**/?(*.)+(spec|test).[jt]s?(x)" ],
+    moduleFileExtensions: ['js']
+};