fix(nextjs): Start navigation transactions on same-route navigations

[lforst]

Fixes #5641

Currently we are not starting transactions in Next.js when navigating from one page to another, but within the same route (eg. from /users/42 to /users/1337, both are on the same route /users/[id]).

This PR fixes this by starting a transaction for those navigations by looking at the window pathname instead. (See code changes for reason why we're not starting transactions when just the query string changes)

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.41 KB (+0.02% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	60.06 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	17.98 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified)	52.92 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	19.77 KB (0%)
@sentry/browser - Webpack (minified)	64.31 KB (0%)
@sentry/react - Webpack (gzipped + minified)	19.79 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	44.71 KB (+0.02% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.9 KB (+0.01% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.27 KB (+0.01% 🔺)