blog: Add tales of open source article: vfscore

Add blog article with the open source history of `vfscore`, the internal
Unikraft library providing the framework for implementing filesystems.

The article timelines the introduction of `vfscore` from OSv's own
`vfs`, which itself was imported from the Prex implementation of `vfs`.

Signed-off-by: Razvan Deaconescu <[email protected]>