Initial commit: Developer Tools MVP with visual editor

- Complete React app with 7 developer tools
- JSON Tool with visual structured editor
- Serialize Tool with visual structured editor
- URL, Base64, CSV/JSON, Beautifier, Diff tools
- Responsive navigation with dropdown menu
- Dark/light mode toggle
- Mobile-responsive design with sticky header
- All tools working with copy/paste functionality

node_modules/eslint-plugin-jest/docs/rules/no-focused-tests.md

@@ -0,0 +1,56 @@
+# Disallow focused tests (`no-focused-tests`)
+
+Jest has a feature that allows you to focus tests by appending `.only` or
+prepending `f` to a test-suite or a test-case. This feature is really helpful to
+debug a failing test, so you don’t have to execute all of your tests. After you
+have fixed your test and before committing the changes you have to remove
+`.only` to ensure all tests are executed on your build system.
+
+This rule reminds you to remove `.only` from your tests by raising a warning
+whenever you are using the exclusivity feature.
+
+## Rule Details
+
+This rule looks for every `describe.only`, `it.only`, `test.only`, `fdescribe`,
+and `fit` occurrences within the source code. Of course there are some
+edge-cases which can’t be detected by this rule e.g.:
+
+```js
+const describeOnly = describe.only;
+describeOnly.apply(describe);
+```
+
+The following patterns are considered warnings:
+
+```js
+describe.only('foo', () => {});
+it.only('foo', () => {});
+describe['only']('bar', () => {});
+it['only']('bar', () => {});
+test.only('foo', () => {});
+test['only']('bar', () => {});
+fdescribe('foo', () => {});
+fit('foo', () => {});
+fit.each`
+  table
+`();
+```
+
+These patterns would not be considered warnings:
+
+```js
+describe('foo', () => {});
+it('foo', () => {});
+describe.skip('bar', () => {});
+it.skip('bar', () => {});
+test('foo', () => {});
+test.skip('bar', () => {});
+it.each()();
+it.each`
+  table
+`();
+test.each()();
+test.each`
+  table
+`();
+```