testing of client and server code with vitest

[benmccann]

test:unit:client could run .svelte and .svelte.js/.svelte.ts files with jsdom and svelte testing library (which sets resolve: browser)

test:unit:server could run the remaining files without those

[benmccann]

Feedback from the vitest team was to use their workspaces feature for this: https://github.com/dominikg/vitest-example-svelte5/blob/main/vitest.workspace.ts