Feature: ability to load an AlphaFold structure and display visually the plddt score

[0gust1]

The standard Molstar Viewer (React-based) does this, but in a way that is not compatible with the current library technology (Svelte) and approach (composable components toolbox).

Refs:

• QualityAssessmentPLDDTPreset and its usage: https://github.com/molstar/molstar/blob/fb16cd0070846fa4e42121961f888d5e73d3876c/src/apps/viewer/app.ts#L15C10-L15C38
• The dedicated loadAlphaFoldDb function and its behavior. https://github.com/molstar/molstar/blob/fb16cd0070846fa4e42121961f888d5e73d3876c/src/apps/viewer/app.ts#L303