update build and deploy tasks to work with SvelteKit

[ryanatkn]

SvelteKit fits nicely with the deployment process we've been doing. Gro was building production artifacts to dist/, SvelteKit outputs to build/.

The trick now is to do the best integration with what src/deploy.task.ts was doing for our static deployments to GitHub Pages.

• basic behavior

So far this PR:

• makes the build and deploy tasks work for SvelteKit projects
• adds custom branch CLI arg to both tasks
• adds the clean CLI arg to exit directly after cleaning up the git and Gro state
• adds the helper hasSvelteKitFrontend and uses it to try to do the right thing
• changes the deployment branch from gh-pages to deploy - we don't want to tie ourselves to GitHub, and now GitHub supports customizing the deployed branch name
• uses SvelteKit's static/ to replace similar functionality we had for things like the CNAME file