Feat: Add ability to have all variables defined at the root level for out-of-the-box experience

[NickDJM]

Summary

Having all of the variables fallback to everything is great for when you want absolute control over everything, but sometimes you want to reference custom props without having to go through sass imports.

Solution

Add a "custom-props" file that defines literally every variable possible in graupl.

Alternatives

Have each component have a custom-props file that can be referenced if needed (this might be a better option)

[NickDJM]

As an example, there would be a file with this in it:

:root {
  --graupl-border-width: 0.125rem;
  --graupl-border-top-width: var(--graupl-border-width);
  --graupl-border-bottom-width: var(--graupl-border-width);
  --graupl-border-left-width: var(--graupl-border-width);
  --graupl-border-right-width: var(--graupl-border-width);
}

People could then load this to have a better experience when trying to use Graupl without having to compile their own thing.