os: add input_password(prompt) and unit tests (#15507)

vlib/os/password_windows.c.v

@@ -0,0 +1,26 @@
+module os
+
+#include <windows.h>
+
+// input_password prompts the user for a password-like secret. It disables
+// the terminal echo during user input and resets it back to normal when done.
+pub fn input_password(prompt string) !string {
+	if is_atty(1) <= 0 || getenv('TERM') == 'dumb' {
+		return error('Could not obtain password discretely.')
+	}
+
+	std_handle := C.GetStdHandle(C.STD_INPUT_HANDLE)
+	mut mode := u32(0)
+
+	unsafe { C.GetConsoleMode(std_handle, &mode) }
+	unsafe { C.SetConsoleMode(std_handle, mode & (~u32(C.ENABLE_ECHO_INPUT))) }
+
+	defer {
+		unsafe { C.SetConsoleMode(std_handle, &mode) }
+		println('')
+	}
+
+	password := input_opt(prompt) or { return error('Failed to read password') }
+
+	return password
+}