Use `pnpm` or update `package.json`

[baseballyama]

dominikg is started to implement svelte-ecosystem-ci and it includes this ESLint plugin also.

For this CI to be successful, either of the following must be implemented

• Use pnpm
• Add "packageManager":"yarn@version" to package.json

I personally want to use pnpm because now both Svelte and SvelteKit is using pnpm.
Then we can think that we can always use pnpm for Svelte repositories.
And I think pnpm is much faster...

[ota-meshi]

I think it's a good idea to use organization's familiar package manager! Let's change ours to use pnpm too.

[baseballyama]

close this because #477 is merged.