ref(tracing): Clarify naming in BrowserTracing integration

[lobsterkatie]

Literally nothing but renames, with a few goals:

1. Bring function naming more in line with what we do elsewhere in the SDK. Specifically, make function names more "verby," so that it's clearer for the reader what's an actor and what's a thing being acted upon.
2. Disambiguate keys and values. I have been careful not to rename the publicly-available routingInstrumentation integration option anywhere. I have, however, renamed our placeholders for that option's value.
3. Disambiguate the transaction-starting function passed to the routing instrumentation from its namesake Sentry.startTransaction.
4. Admittedly this currently only matters to me, but doing this was a way to carefully examine the routing instrumentation code, which is helpful since I'm about to build a version of it for next.js.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	20.45 KB (-0.01% 🔽)
@sentry/browser - Webpack	21.29 KB (0%)
@sentry/react - Webpack	21.33 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	27.53 KB (-0.01% 🔽)