rust port: expose an API to use `metacall_clear`

[fahdfady]

feature

there is no API to unload scripts from metacall in the Rust port.
this will be helpful:

core/source/ports/rs_port/src/bindings.rs

Lines 1389 to 1392 in 057d988

	unsafe extern "C" {
	#[doc = " @brief\n Clear handle from memory and unload related resources\n\n @param[in] handle\n Reference to the handle to be unloaded\n\n @return\n Zero if success, different from zero otherwise"]
	pub fn metacall_clear(handle: *mut ::std::os::raw::c_void) -> ::std::os::raw::c_int;
	}

motivation

I needed that when i was developing https://github.com/fahdfady/metacall-playground/ and didn't find a way to achieve this functionality through the metacall crate (0.5.2)