Refactor main menu JS & styling

[AccessPaul]

While updating the Wilson demo site, I discovered that the main menu javascript was very broken, and stupidly complex. This PR refactors the JS and styling for the main menu to fix the issues I spotted.

I rewrote the JS to use use Drupal once so that click events wouldn't be fired twice, which was the main reason why the menus weren't opening correctly. I also reduced some complexity by making makeActive and makeInactive functions that help reduce repeated code.

@ismael-abujadur I'm raising this PR against Wilson, but I'd strongly recommend you copy the updated files in to your custom WaterUK theme before you get too far along with the customisation.

[AccessPaul]

Adding a video of the latest version of this. Changes include:-

• Make the mobile menu overlay appear below the sticky mobile header when open. Essentially how the WaterUK site has been designed.
• Created a "hamburger" icon that toggles menu open/closed and animates in to a X icon when active.
• Refactoring the JS code to make more efficient use of events and reusable functions.
• Standardise the is-active classes across the various elements of navigation.

https://user-images.githubusercontent.com/1227987/221359844-9eb7b51e-1e69-4eae-9034-5e754d982c76.mov