feat(ci): Add Node 18 to test matrix

[AbhiPrasad]

https://nodejs.org/en/blog/announcements/v18-release-announce/

From reviewing the CHANGELOG (https://github.com/nodejs/node/blob/master/doc/changelogs/CHANGELOG_V18.md), there seems to be no changes that will affect us in the code.

See v17 CHANGELOG: https://github.com/nodejs/node/blob/master/doc/changelogs/CHANGELOG_V17.md

Notable things we can maybe pursue:

• Make a Node 18 only fetch transport: https://github.com/nodejs/node/blob/master/doc/changelogs/CHANGELOG_V18.md#fetch-experimental

Resolves https://getsentry.atlassian.net/browse/WEB-870

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.41 KB (-3.61% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	60.11 KB (-6.97% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.26 KB (-3.19% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	53.8 KB (-7.2% 🔽)
@sentry/browser - Webpack (gzipped + minified)	20 KB (-13.92% 🔽)
@sentry/browser - Webpack (minified)	63.34 KB (-22.49% 🔽)
@sentry/react - Webpack (gzipped + minified)	20.03 KB (-13.96% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	43.5 KB (-9.48% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.1 KB (-3.76% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	23.63 KB (-3.49% 🔽)

[timfish]

Make a Node 18 only fetch transport

My main gripe with fetch on node.js is that you appear to lose proxy/ca options!