PHP, SCSS and JS linting tools for use in local and Travis

[Ismail-elkorchi]

Changes proposed in this Pull Request:

This PR updates the linting tools used in Travis by :

• Adding package.json file and specifying @wordpress/scripts as the only dependency.

• Adding composer.json file and specifying Composer installer for PHP_CodeSniffer, WPThemeReview and PHP Parallel Lint as dependencies

• Adding lint:scss command to package.json to lint scss files using the lint-style script which is defined in @wordpress/scripts

• Adding lint:js command to package.json to lint js files using the lint-js script which is defined in @wordpress/scripts

• Adding lint:wpcs command to composer.json to lint php files using php_codesniffer

• Adding lint:php command to composer.json to lint php files for syntax errors

• Adding .stylelintrc.json and .eslintrc configuration files for stylelint and eslint

• Removing .jshintignore and .jscsrc files since @wordpress/scripts uses eslint under the hood

• Updating .travis.yml to use newer versions of php, install the dependencies and run the commands defined in package.json and composer.json

• Updating phpcs.xml.dist

With these changes Travis is now reporting many issues regarding the coding standards which must be fixed in https://github.com/Automattic/_s/pull/1388.

Related issue(s):

https://github.com/Automattic/_s/pull/1311, https://github.com/Automattic/_s/issues/1149, https://github.com/Automattic/_s/pull/1159, https://github.com/Automattic/_s/pull/1291