[Feature Request]: Add filter() method to PCollection in TypeScript SDK

[junaiddshaukat]

What would you like to happen?

Summary

Add a filter() method to the PCollection class in the TypeScript SDK, consistent with the existing map() and flatMap() methods.

Motivation

The TypeScript SDK currently provides map() and flatMap() methods on PCollection, but lacks a filter() method. This is explicitly mentioned as a TODO in the README-dev.md:

"Also add a [do]Filter, and possibly a [do]Reduce?"

The Python SDK already provides beam.Filter() for this purpose. Adding filter() to the TypeScript SDK would:

1. Improve developer experience - JavaScript developers expect filter() alongside map()
2. Improve code readability - pcoll.filter(x => x > 0) is cleaner than pcoll.flatMap(x => x > 0 ? [x] : [])
3. Maintain consistency with the Python SDK which has beam.Filter()

Proposed API

// Filter even numbers
const evens = pcoll.filter(x => x % 2 === 0);

// Filter with context (for side inputs, etc.)
const filtered = pcoll.filter((x, ctx) => x > ctx.threshold, { threshold: sideInput });

Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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[junaiddshaukat]

.take-issue