Add wrappers for XPLMGetSystemPath

[JDeeth]

Currently there's no wrapper for XPLMGetSystemPath

https://developer.x-plane.com/sdk/XPLMGetSystemPath/
https://docs.rs/xplm-sys/latest/xplm_sys/fn.XPLMGetSystemPath.html

I'm a bit unsure how to use this function safely in Rust given it's a string pointer outparam!

[JDeeth]

With thanks to the "No Boilerplate"/Namtao Discord community, how to outparam a string:

let mut buffer = &mut [0_u8; 512];
XPLMGetSystemPath(buffer);
let s = CStr::from_bytes_until_nul(buffer)?.to_str()?.to_string();

I can go ahead now, and should be able to get a pull request in later this week

[mkreu]

Hey there, sounds good to me!

I will have a look at your PRs on the weekend, once I am back from vacation :)