Add window['_fs_ready'] callback

[aleksandergurin]

In FullStory there is a useful callback that you can set window['_fs_ready'] = function() {};.

Why that callback is useful? There you can call FS.identify(uid, userVars?) to identify a user or you can get current session URL.

Some documentation:

• https://developer.fullstory.com/callbacks-and-delegates
• https://help.fullstory.com/hc/en-us/articles/360020828073-FS-getCurrentSessionURL-Retrieving-a-session-URL-at-time-of-recording

[luser]

Hi there! Thanks for this PR, I'm sorry we haven't been very attentive to contributions on this repository. Unfortunately we can't make changes to src/snippet.js in this repository, as it's a generated file that's synced from elsewhere. I'm going to add some things to the repository to make that clearer to avoid this in the future.

Additionally, there's a newer API that has not yet been made generally-available that allows registering multiple listeners, similar to using window.addEventListener('load', …)instead of window.onload = …. In light of that, I'm going to close this PR and open a new issue for this functionality.