To allow anything to depend on WASI types, including platform-specific
data structures, move the WASI libc filesystem/clock interface into
`platform_api_extension.h`, which leaves just WASI types in
`platform_wasi.h`. And `platform_wasi.h` has been renamed to
`platform_wasi_types.h` to reflect that it only defines types now and no
function declarations. Finally, these changes allow us to remove the
`windows_fdflags` type which was essentially a duplicate of
`__wasi_fdflags_t`.
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