Add IO/FS/flush function to prelude

[developedby]

HVM now exposes a file flushing primitive (HigherOrderCO/HVM#392), so we should create a function in the bend prelude to expose it to the users, like is done for the other file primitives.

[bhavik-goplani]

@developedby Hey, I'd like to contribute to Bend. Do you think I can try this issue?

[developedby]

Of course!
If you look at https://github.com/HigherOrderCO/Bend/pull/593/files, you can see more or less what you need to do, just update builtins.bend, builtins.md and changelog.md.
The function should be called IO/FS/flush, receive a file descriptor as argument and return nothing, like IO/FS/write returns nothing.

To test it you have to use the main branch from HVM since flush is not in a rleased version yet

[bhavik-goplani]

Sounds good, thanks!

[developedby]

Closed by #619