Added Terminal.getTerminalSize().

src/main/java/net/rubygrapefruit/platform/internal/MutableTerminalSize.java

@@ -0,0 +1,16 @@
+package net.rubygrapefruit.platform.internal;
+
+import net.rubygrapefruit.platform.TerminalSize;
+
+public class MutableTerminalSize implements TerminalSize {
+    int rows;
+    int cols;
+
+    public int getCols() {
+        return cols;
+    }
+
+    public int getRows() {
+        return rows;
+    }
+}

src/main/java/net/rubygrapefruit/platform/internal/PosixTerminalFunctions.java

@@ -1,5 +1,7 @@
 package net.rubygrapefruit.platform.internal;
 
 public class PosixTerminalFunctions {
-    public static native boolean isatty(int fildes);
+    public static native boolean isatty(int filedes);
+
+    public static native void getTerminalSize(int filedes, MutableTerminalSize size, FunctionResult result);
 }