added SpellingDictionaryCollection to cspell-lib exports

[wujekbogdan]

I'm trying to use cspell-lib in our application (we're building a spell-checker service in node - more details here: #1813).

I need to use the SpellingDictionaryCollection class to be able to add custom dictionaries, but this class is not exported from cspell-lib. The only way to import it is:

import { SpellingDictionaryCollection } from 'cspell-lib/dist/SpellingDictionary';

But this isn't a good solution as it causes Webpack builds to fail because of non-static require() calls.

This PR adds SpellingDictionaryCollection to packages/cspell-lib/src/index.ts exports, thanks to this it can be imported this way:

import { SpellingDictionaryCollection } from 'cspell-lib';