Performance: Add Bundlesize monitoring

Created by: umpox

This will give us some raw bundlesize comparisons which should be used alongside Lighthouse and will give us more fine-grained bundle analysis that isn't as vulnerable to varying results like Lighthouse is

Success

Success status check:

Failure

Failed status check:

Details shows the exact bundle(s) that failed:

Buildkite pipeline will also fail with details:

Closes https://github.com/sourcegraph/sourcegraph/issues/26282