Adds detection and automatic wiring for the WGSL texture_and_sampler_let language feature, which allows assigning texture and sampler variables to let bindings within a WGSL shader:

let tex = myTexture;
let s = mySampler;
let c = textureSample(tex, s, uv);

Chrome describes it as preparation for bindless-style support — once textures/samplers can be stored in locals, shaders can do indirection/selection patterns that today require duplicated code paths.

Follows the same pattern as unrestricted_pointer_parameters (#8785), pointer_composite_access (#8786), and packed_4x8_integer_dot_product (#8787).

Changes:

• GraphicsDevice.supportsTextureAndSamplerLet flag, probed from navigator.gpu.wgslLanguageFeatures in WebgpuGraphicsDevice#initDeviceCaps.
• CAPS_TEXTURE_AND_SAMPLER_LET shader define for conditional compilation (#ifdef).
• requires texture_and_sampler_let; directive automatically injected into WGSL shaders on supporting devices via ShaderDefinitionUtils.getWGSLEnables.

Notes:

• Infrastructure-only — no shader callers yet. The cap unlocks future bindless-style helpers / indirection patterns.
• On devices that don't expose the feature, the cap is false and no requires directive is emitted, so existing portable shaders continue to compile.

See https://developer.chrome.com/blog/new-in-webgpu-146