We currently serve icons in the form of 141 separate svg files. This is
pretty inefficient: it results in 141 http requests each with its own
overheads and costs. Each one is generally less than 1kb of data.
This PR merges all of our icons into a single 44kb icon file. Each item
in the svg has an ID, and we use the ID in the asset url to target the
specific icon. This also fixes a lot of the icon preloading issues:
because the icon file is already loaded, there's no extra request needed
when panels open etc.
I was messing with this whilst killing time before a late meeting so
made a shitty overengineered code generator, dont at me
### Change type
- [x] `improvement`
### Release notes
- Serve icons more efficiently, and make sure they're still available if
tldraw goes offline.
