feat: impl reference

[seokju-na]

Add reference feature.

import { openRepository } from 'es-git';

const repo = await openRepository('...');
const ref = repo.getReference('HEAD');
console.log(ref.type()); // 'Symbolic'
console.log(ref.symbolicTarget()); // 'refs/heads/main'

const resolvedRef = ref.resolve();
console.log(resolvedRef.type()); // 'Direct'
console.log(resolvedRef.isBranch()); // true
console.log(resolvedRef.target()); // 'a01e9888e46729ef4aa68953ba19b02a7a64eb82'