chore(build): Remove default @rollup/plugin-node-resolve option

[lobsterkatie]

According to the @rollup/plugin-node-resolve docs, the mainFields option tells the plugin what field in package.json to use as an entry point. The default value is ['module', 'main'], and the plugin will use the first element of the array for which it finds a match in package.json.

All of our package.json files have a module entry, which is reflected in the fact that we've had mainFields set to ['module']. But that of course leads to the same outcome as the just using the default would - there's a match on 'module', and so that's what's used. This PR therefore removes the option in favor of using the default. As would be expected, there is no change in the bundle content.

[github-actions]

size-limit report

Path	Base Size (32edb0b)	Current Size	Change
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.77 KB	19.77 KB	0%
@sentry/browser - ES5 CDN Bundle (minified)	63.47 KB	63.47 KB	0%
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.44 KB	18.44 KB	+0.01% 🔺
@sentry/browser - ES6 CDN Bundle (minified)	56.6 KB	56.6 KB	0%
@sentry/browser - Webpack (gzipped + minified)	22.2 KB	22.2 KB	0%
@sentry/browser - Webpack (minified)	76.36 KB	76.36 KB	0%
@sentry/react - Webpack (gzipped + minified)	22.23 KB	22.23 KB	0%
@sentry/nextjs Client - Webpack (gzipped + minified)	46.4 KB	46.4 KB	0%
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	27.23 KB	27.23 KB	+0.01% 🔺