Sync with svelte CLI fork

[43081j]

svelte currently have their own fork of clack so they could patch certain things.

A quick bit of git-fu shows me that clack core only has the following differences:

• return types to satisfy their isolatedModules setting
• non-null assertions to satisfy their linter/tsconfig (i.e. foo.bar!)
• selectableGroups in group multi-select
  • this one has no git history on their end, it just existed from the start. we don't seem to have a trace of it either, so it does seem to be a feature they added
  • it basically disallows selecting the group options in the select

[natemoo-re]

Thanks for digging into this! I'll start working on implementing these.

[43081j]

I have a branch with the differences in core but almost all of it was just return types, linting, etc

So you're probably still better off making a new branch with only the selectableGroups feature

[natemoo-re]

Looks like #199 is the source of this feature, but they ended up not using it after all. Still seems nice to add! #199 (comment)