ref(tracing): Clean up various things before fixing sampling

[lobsterkatie]

Extracted from #2921, to try to make it less of an unwieldy mess. Random refactoring of stuff - mostly tests - not directly related to those changes, but which came as a consequence of working on that.

I believe the only minorly substantive changes are:

• We had a default beforeNavigate which did nothing (in order that the option be able to be required), which I removed in favor of letting the option be optional and only running beforeNavigate if it's defined.
• I removed the Response type since it's a dom type and not everyone compiles with those.
• For XHR requests, the method and url are now added to the span when it's created rather than when it's finished, for consistency with fetch requests.

There's a bunch here, but I did my best to keep my commits clean* so it would be possible to look at each one individually and make sure I'm not doing anything nuts or breaking.

*only about one thing, named appropriately, with passing tests at each stage

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	18.01 KB (+0.01% 🔺)
@sentry/browser - Webpack	18.82 KB (0%)
@sentry/react - Webpack	18.82 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	23.81 KB (-0.03% 🔽)

[kamilogorek]

It'll fix #2943 as well