Add new insight audit: CharacterSet

[connorjclark]

See core/scripts/generate-insight-audits.js which currently excludes this.

During a breaking change, would then remove charset audit.

[minhbuild]

Hi! My teammate and I are CMU CS students working on OSS as part of a course project for Foundations of Software Engineering (17-313). Here's my understanding of the work involved for this task:

1. Remove CharacterSet from the exclusion list in core/scripts/generate-insight-audits.js and run yarn generate-insight-audits to scaffold the boilerplate file under core/audits/insights/.
2. Implement the audit in the generated file using the adaptInsightToAuditProduct helper (the same pattern used by existing insight audits like image-delivery-insight and lcp-phases-insight), wiring it up to the CharacterSet insight model from the trace engine.
3. The existing charset audit would stay in place for now and be removed in a future breaking change, as noted in the issue.

Is there anything non-obvious about the CharacterSet insight model that would affect how adaptInsightToAuditProduct needs to be called, compared to the simpler existing implementations? Should the new audit be hidden in the UI (like the original blank implementations were), or is it ready to be surfaced?

Thanks!