feat: add stale-if-error on served packages

[martcl]

Issue:

• Update Cache-Control with stale-while-revalidate/stale-if-error directives. #1257

Problem:
When esm.sh goes down, it is not realistically able to survive on cache. This is due to how the cache-control header is configured and how a lot of packages don't define a strict version for their dependencies. When requesting a package from esm you are able to define a strict version, but you don't control the versioning of transitive packages. They could be defined without a strict version, e.g ^0.27.0. This transitive package is required for your package to work, but is only cached 10 minutes, making fact that a striced versioned package is cached for a day not a good mechanism for surviving esm.sh downtimes.

Fix:
Adding stale-if-error directives to the cache-control header will give proxy/caches in front the ability to serve the content while the service is unavailable.

Change:

• Add stale-if-error=86400 on cache-control header used to serve esm packages. This allows for a day worth of downtime.

[filipgolonka]

Nice!

[ije]

thanks

[midnight-wonderer]

Please ensure that Do not serve stale content while updating is set to off in Cloudflare.
I'm not sure if this affects stale-if-error, but it definitely affects stale-while-revalidate in my tests.

Also, it would be nice if we could address decision regarding asynchronous revalidation (stale-while-revalidate) in the original thread, as this PR does not cover that case.