Add support for filesize and similar expressions

[MatejKastak]

We do not support the built-in symbols that are not from modules. The other symbol is deprecated entrypoint.

• they should be documented here

#[test]
fn test_eval_filesize() {
    let mut context = common::context_with_pe_sample_and_rule();
    let res = context.eval("filesize").unwrap();
    assert_eq!(res, YrValue::Integer(123));
}

Make sure this is also supported in complex expressions when comparing against 1200KB for example.

[MatejKastak]

Looks like mapping it directly to YrValue::Integer is not possible since filesize is u64 and integer is i64 like the underlying YARA VALUE. There was a bug where it was not possible to evaluate complex conditions with filesize. That is fixed for now.