Move to monorepo

[A1Liu]

Set up the project as a monorepo

Notes:

• The only actually useful guide I found for this was this: https://vedanshmehra.hashnode.dev/setting-up-a-monorepo-with-vite-typescript-and-pnpm-workspaces#heading-setup-with-pnpm-workspaces , everything else said that you had to use turborepo, which... is a lot. It sorta frustrates me that the only solution that consistently seems to work for JS/TS dev speedups is to cache and save literally everything ever.
• The thing that I actually used the guide for was to get the confidence to actually try to put sveltekit in a monorepo, because I kept seeing that it was infeasible or difficult. I guess time will tell whether there's some hidden dependency problem with sveltekit that I haven't yet found.