ref(utils): Extract out Object.prototype.toString into var

[AbhiPrasad]

Save bundle size by not repeating prototype identifier

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[github-actions]

size-limit report

Path	Base Size (5216184)	Current Size	Change
@sentry/browser - CDN Bundle (gzipped)	22.45 KB	22.46 KB	+0.06% 🔺
@sentry/browser - Webpack	23.29 KB	23.3 KB	+0.08% 🔺
@sentry/browser - Webpack - gzip = false	82.73 KB	82.61 KB	-0.15% 🔽
@sentry/react - Webpack	23.32 KB	23.33 KB	+0.07% 🔺
@sentry/nextjs Client - Webpack	48.07 KB	48.08 KB	+0.01% 🔺
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	29.98 KB	29.99 KB	+0.03% 🔺