docs: add tsimportDirectory to README

[Copilot]

tsimportDirectory was not documented in the README despite being a public export.

Changes

• Added "Import Directory" section covering tsimportDirectory usage, including a migration-pattern example and notes on named/null export variants — consistent with how tsimport options are already documented.

import { tsimportDirectory } from 'ts-import-ts';

// Load default export from every file in the directory
const migrations = tsimportDirectory('migrations');
for (const migration of migrations) {
  migration();
}

// Load a named export from each file
const items = tsimportDirectory('my-dir', 'myExport');

// Load all exports from each file
const allItems = tsimportDirectory('my-dir', null);