Add plyToTurn util function

It's defined in three different places in lila.

src/util.test.ts

@@ -1,5 +1,5 @@
 import { expect, test } from '@jest/globals';
-import { makeUci, parseUci } from './util.js';
+import { makeUci, parseUci, plyToTurn } from './util.js';
 
 test('parse uci', () => {
   expect(parseUci('a1a2')).toEqual({ from: 0, to: 8 });
@@ -12,3 +12,11 @@ test('make uci', () => {
   expect(makeUci({ from: 2, to: 3 })).toBe('c1d1');
   expect(makeUci({ from: 0, to: 0, promotion: 'pawn' })).toBe('a1a1p');
 });
+
+test('plyToTurn', () => {
+  expect(plyToTurn(1)).toBe(1);
+  expect(plyToTurn(2)).toBe(1);
+  expect(plyToTurn(3)).toBe(2);
+  expect(plyToTurn(4)).toBe(2);
+  expect(plyToTurn(5)).toBe(3);
+});

src/util.ts

@@ -39,6 +39,8 @@ export const roleToChar = (role: Role): string => {
   }
 };
 
+export const plyToTurn = (ply: number): number => Math.floor((ply - 1) / 2) + 1;
+
 export function charToRole(ch: 'p' | 'n' | 'b' | 'r' | 'q' | 'k' | 'P' | 'N' | 'B' | 'R' | 'Q' | 'K'): Role;
 export function charToRole(ch: string): Role | undefined;
 export function charToRole(ch: string): Role | undefined {