perf(core): Prevent creation of new contexts object on scope

[AbhiPrasad]

Based on feedback from #6154 (comment)

Remove spread operator usage when setting new context so we mutate the current reference instead of creating a new object. Should reduce bundle size and be slightly more performant.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.5 KB (+0.03% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	60.34 KB (-0.06% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.15 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	53.69 KB (-0.03% 🔽)
@sentry/browser - Webpack (gzipped + minified)	19.89 KB (-0.01% 🔽)
@sentry/browser - Webpack (minified)	65.1 KB (-0.04% 🔽)
@sentry/react - Webpack (gzipped + minified)	19.91 KB (-0.01% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	45.88 KB (+0.01% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.3 KB (+0.02% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.7 KB (+0.01% 🔺)