fix(zone.js): allow enabling default `beforeunload` handling

[arturovt]

Prior to this commit, when zone.js was included, it wasn't possible to handle beforeunload events correctly if event handlers returned strings to prompt the user.

With this change, we introduce a global configuration flag, __zone_symbol__enable_beforeunload, to allow consumers to enable the default beforeunload handling behavior.

This flag is necessary to prevent any breaking changes resulting from this modification. The previous attempt to fix it caused a large number of failures in G3. Hence, we're hiding that fix behind the configuration flag.

Closes #47579 Closes #52256