feat: filtering pointer by language tag

[tpluscode]

• Extends filter to allow accessing all pointers.
• Exports a filter function to select only literals matching given language tag(s)

import { taggedLiteral } from 'clownface/filter'

let allLiteral: MultiPointer

const germanLiterals = allLiterals.filter(taggedLiteral(['de-CH', 'de']))

• [ ] update documentation