Split sprites in individual svg files

Currently, svg icons are provides as a theme dependent sprites (default and hsl in this repo, addtional ones in downstream repositories). They are hard to maintain and contain much duplication.

For the embark-okc fork, we split them into indiviual svg files, and recreate the sprites in the static build step.

I suggest to do this upstream as well, as these individual svgs are easier to update, easier to override by inheriting themes, and could make of use includes to reduce duplication.

Should I provide a PR for this or do you have other plan?

Another question is what the long term strategy for icons is. digitransit-component-icon also duplicates many icons, but it is not evident, when to use this in contrast to the sprites approach, and how to support different themes.

Start by reviewing the two linked commits: one splits the sprites into individual SVG files, and the other recreates sprites during the static build step. Then inspect how this repository uses its default and hsl sprites alongside digitransit-component-icon. Done requires an agreed upstream icon strategy and an implementation that avoids duplication while supporting inherited themes.