Added filterUndefined params to array rule

[vrag86]

I add filterUndefined params to array rule. It need when create custom items validator and skip not validated items

[icebob-ai]

Thanks for the PR, @vrag86!

The idea of filtering values from arrays is useful, but there are a few issues to address:

Missing pieces

• No tests — need at least: filterUndefined removes undefined, doesn't remove null/falsy, works with items validation, disabled by default
• No TypeScript type updates — RuleArray in index.d.ts needs a filterUndefined?: boolean property
• No README documentation for the new option

Design concern

A more universal approach would be better here. Instead of only filtering undefined, consider a more general option like filter that could handle multiple cases:

• Filter undefined values
• Filter null values
• Filter both null and undefined (nullish)
• Or even a custom filter function

Something like filter: "undefined" | "null" | "nullish" or filter: (value) => boolean would be more flexible and cover more use cases.

Please consider reworking this with a broader design, adding tests, TypeScript types, and documentation.