Add flag to browsingContext.close to skip beforeunload

[Lightning00Blade]

Original issue #170, Original discussion comment - #170 (comment)
The issue was no discussed in depth as that focused more on #187
Currently when we close a BrowsingContext we need to run close-a-top-level-traversable in there check if unloading is canceled.
From an user perspective I may want to close the BrowsingContext to free resources regardless if there is unload handlers or not.

Puppeteer CDP already supports this at Page.close,
where default is to skip beforeunload.