Add BigCommerce CDN image provider support

[stefashkaa]

Feature Title

Add BigCommerce CDN image provider support

Description

Add a width-only BigCommerce CDN provider based on the image URL template contract used by the official Catalyst loader.

Use Case

A BigCommerce storefront receiving CDN URLs with a {:size} placeholder needs width-based responsive sources through the package.

Proposed Solution

• Add and export bigcommerce through the core provider registry so Angular, React, and Svelte can use the shared configuration.
• Accept documented templated image URLs and replace {:size} with the requested ${width}w size token.
• Preserve the rest of the URL, including existing metadata and parameters, and define clear behavior for URLs without the required placeholder.
• Keep the initial integration width-only. Do not claim height, quality, or explicit format support from the Catalyst example; constrain those requests so unsupported picture formats are not advertised.
• Add setup documentation and focused fixtures covering multiple responsive widths, placeholder replacement, missing placeholders, and unsupported modifier behavior. Validate advertised dimensions and content types against an enabled service or controlled server where applicable.

Alternatives Considered

Use Catalyst’s CDN loader directly or substitute the size token in application code. A core provider would integrate the same width contract with responsive source generation.

Examples from Other Libraries

The official BigCommerce Catalyst CDN image loader implements the size-token substitution.