fix(nextjs): Silence warning about usage of process in runtimes where it is not available

[lforst]

The isBrowser function is not used anywhere but accesses process and makes Next.js print a warning since process is not available in the Edge runtime.

This PR deprecates the function (because it is unused) and uses global.process instead which seems to outsmart Next.js' static code analyzer.

Also added tests to prevent stuff like this in the future: #9382

[Lms24]

isBrowser is used in Replay:

sentry-javascript/packages/replay/src/integration.ts

Lines 215 to 217 in 6d9a766

	if (this._isInitialized && isBrowser()) {
	throw new Error('Multiple Sentry Session Replay instances are not supported');
	}

Do you see another way how we could keep such a check but avoid accessing process or other APIs? I'm not totally opposed to deprecating/eventually removing this function but I can see such a check being useful for full stack/SSR frameworks or stuff like electron/cordova/capacitor...

[lforst]

isBrowser is used in Replay

@Lms24 Dang how could I miss that... I guess I'll undeprecate it? The GLOBAL_OBJ thing still works in the case of Next.js.