nice, Windows: support mapping current Priority Class

[oech3]

While setting Priority Class is supported and Priority Class is mapped like

coreutils/src/uu/nice/src/nice.rs

Lines 159 to 167 in 2c0e1b9

	#[cfg(windows)]
	let priority_class = match new_niceness {
	..=-20 => REALTIME_PRIORITY_CLASS,
	-19 => HIGH_PRIORITY_CLASS,
	-18..=-1 => ABOVE_NORMAL_PRIORITY_CLASS,
	0 => NORMAL_PRIORITY_CLASS,
	1..=18 => BELOW_NORMAL_PRIORITY_CLASS,
	19.. => IDLE_PRIORITY_CLASS,
	};

. showing (mapping) current Priority Class is not supported:

coreutils/src/uu/nice/src/nice.rs

Lines 109 to 110 in 2c0e1b9

	#[cfg(not(unix))]
	let current_niceness = 0i32; // todo: what we can do?

It would be better to be compatible with Cygwin's nice command.
Also, nice should show raw Priority Class e.g. instead of value if it is not a mapped class.
ref: https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-setpriorityclass