Add ShortPixel Adaptive Images image provider support #40

Feature Title

Add ShortPixel Adaptive Images image provider support

Description

Add a ShortPixel Adaptive Images provider that represents its path options and categorical quality controls explicitly.

Use Case

An application using ShortPixel’s adaptive image service needs responsive URLs while selecting native compression behavior and first-request handling.

Proposed Solution

• Add and export shortpixel through the core provider registry so Angular, React, and Svelte can use the shared configuration.
• Build documented /client/{options}/{source-url} URLs, including options such as w_800,h_600,q_lossy,to_auto,ret_wait.
• Expose native quality values such as lossless, glossy, and lossy as provider-specific options. Do not silently translate arbitrary numeric quality into categories without a documented policy.
• Preserve source URLs and expose documented ret behavior. Account for automatic conversion and browser-dependent fallback when defining which explicit picture formats can be supported.
• Add setup documentation and focused fixtures covering native quality options, source encoding, first-request controls, and negotiated-format fallback. Validate advertised dimensions and content types against an enabled service or controlled server where applicable.

Alternatives Considered

Use ShortPixel’s integration directly or build URLs in a custom provider. Numeric-quality adapters do not capture its named compression modes without additional mapping rules.

Examples from Other Libraries

• Not found
• Adaptive Images API parameters