More specific return types in Format#readFeatures

[ahocevar]

This pull request improves the types of the GeoJSON and MVT format so they can infer the return type of readFeatures() from the featureClass constructor option. This means that no type casts are needed any more in the code snippets below:

/** @type {Array<Feature>} */
const features = new GeoJSON().readFeatures(source);

/** @type {Array<RenderFeature>} */
const renderFeatures = new GeoJSON({featureClass: RenderFeature}).readFeatures(source);

For all other formats, the Feature return type is used for readFeatures(), as before v8.2.0.

Fixes #15334.

[github-actions]

📦 Preview the website for this branch here: https://deploy-preview-15338--ol-site.netlify.app/.

[tschaub]

Nice!