feat: add unregisterFilter method

[YacovGold]

Summary

• Add Liquid#unregisterFilter(name)
• Support removing custom and built-in filters
• Allow filters to be registered again after removal
• Document restoring built-in filters through the exported filters object
• Document behavior with strictFilters

Motivation

This provides a supported API for removing filters without directly modifying the filter registry.

When strictFilters is disabled, an unregistered filter is skipped. When enabled, using it throws an undefined-filter error. Unregistering an unknown
filter is a no-op.

Built-in filters can be restored using the existing registerFilter() API:

import { filters } from 'liquidjs'

engine.registerFilter('plus', filters.plus)

[harttle]

@all-contributors please add @YacovGold for code

[allcontributors]

@harttle

I've put up a pull request to add @YacovGold! 🎉

[github-actions]

🎉 This PR is included in version 10.29.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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