I've been trying to use the "mark scrap that contains xxx items" tool to essentially highlight all takeable scrap in the wasteland, but with no luck. Is there a way to mark every single kind of material, so that all scrap would be highlighted? If this doesn't exist in the default game, can it be easily modded/has already been modded?
I'm not sure why you would want to do this, honestly, but it is doable, more or less.
It sounds like you're familiar with marking loot based on its components. To achieve your goal, you'll simply need to mark every possible component.
Unfortunately, you can only mark components that you have in your inventory, or that are needed to build something in a crafting station or the workshop. Therefore, it will take some time to mark everything, as you'll have to acquire at least one item containing that component first.
You can find a list of all the components on Nukapedia, if you'd like to check them off as you go. If you click on any component on that list, it will take you to a list of the items that contain that component, making it easier to track down ones you don't have yet.
Or, since nearly everything is scrappable, you could just pick up everything, at least at first. Then, before depositing your junk in your workshop, go into the component view in your inventory and make sure everything is marked.
With the Scrapper 2 perk, this will result in anything you've already marked being highlighted in the world. This will result in everything scrappable being highlighted before long.
This endeavor would be aided by a list of all the junk that is not scrappable. I doubt this list is very long, so you could just memorize what's on it, and know not to pick those things up early when you're trying to mark everything.