Add stat instance method to File.

test/io/file/stat.wren

@@ -0,0 +1,18 @@
+import "io" for File, Stat
+
+var file = File.open("test/io/file/file.txt")
+var stat = file.stat
+
+System.print(stat is Stat)            // expect: true
+System.print(stat.device is Num)      // expect: true
+System.print(stat.inode is Num)       // expect: true
+System.print(stat.mode is Num)        // expect: true
+System.print(stat.linkCount)          // expect: 1
+System.print(stat.user is Num)        // expect: true
+System.print(stat.group is Num)       // expect: true
+System.print(stat.specialDevice)      // expect: 0
+System.print(stat.size)               // expect: 19
+System.print(stat.blockSize is Num)   // expect: true
+System.print(stat.blockCount is Num)  // expect: true
+
+file.close()