This commit abstracts out the specifics of the underlying system calls
FS operations behind a new `FileSystem` and `File` trait in the
`ext/fs` extension.
This allows other embedders to re-use ext/fs, but substituting in a
different FS backend.
This is likely not the final form of these traits. Eventually they will
be entirely `deno_core::Resource` agnostic, and will live in a seperate
crate.
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Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>
