Add categories for mixin list in right panel

[Algorush]

So far I have made it without a design from Figma. I will add styles in the next commit. For this PR to work, first need to apply changes from this PR: https://github.com/3DStreet/3dstreet/pull/418 So that information about mixin groups is stored in the attribute of street-assets element. This is version 1. But I decided to implement the mixin grouping differently. I will store the category name in the category attribute for each mixin. We can also take the group name from the html comments, but this is unreliable for the future. @kfarr Maybe you will comment also which option is better for your opinion?

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[Algorush]

Added new commit to this PR and to PR in 3DStreet repo: https://github.com/3DStreet/3dstreet/pull/418. Now I have changed the way category names are stored, now they are stored in the category attribute for each mixin. And the function for creating groups in the mixin component has also been changed

[kfarr]

this works really well! thanks @Algorush