Add separate Joomla 4 javascript page (#252)

Author: wilsonge

manual/javascript.md manual/javascript-j3.mdmanual/javascript.md renamed to manual/javascript-j3.md

@@ -0,0 +1,20 @@
+### Javascript in Joomla 4
+
+Joomla 4 uses ES6 syntax where ever possible. As part of this change we decided to use an industry standard codestyle
+rules for our javascript - the AirBNB coding standards. These can be found [on their GitHub page](https://github.com/airbnb/javascript#table-of-contents)
+
+We have four modifications to the defaults.
+
+1. We allow parameter reassignment of function parameters to allow easier setting
+of defaults (for examples please see the ESLint rules [here](https://eslint.org/docs/rules/no-param-reassign)).
+
+2. Secondly we only allow the imports of dev dependencies from npm in our build directory (`build/`) (rather than in all
+files).
+
+3. We currently allow browser alerts using the `alert()` function (the long term intention is to disable this as we move
+to our custom element alerts)
+
+4. We always enable javascript strict mode as we aren't using javascript modules at the moment
+
+We also use ESLint to enforce these rules. If you are familiar with javascript from other projects you've worked on you
+can find our eslint rules at (in the following location on GitHub)[https://github.com/joomla/joomla-cms/blob/4.0-dev/.eslintrc]