Add yarn audit step to CI

[antonis]

Part of the dependency pinning audit — #6239 / SDK-1316.

The repo currently has no yarn audit or equivalent step in CI to flag known vulnerabilities in npm dependencies.

Note: this is not an established pattern in Sentry JS-ecosystem SDKs (sentry-javascript doesn't have it either), so this may be better addressed org-wide rather than ad hoc.

[linear-code]

RN-664

[antonis]

Dependabot security alerts are already enabled for the npm ecosystem in this repo, covering the same ground. A separate yarn audit CI step would be redundant.