Add baseline for detect-secrets for Positron (#3550)

Reviewed excludes with team members:
        "scripts/git_hooks/secrets/.secrets.baseline.*",
        ".*cgmanifest.json",
        ".*/html-manager/dist/embed-amd.js",
        "src/vs/base/test/common/filters.perf.data.js",
        ".*/test/browser/recordings/windows11.*"

Reviewed remaining flagged secrets with team members, all marked not a real secret.

TODO: Define pre-commit hook to run detect-secrets audit