Add index parameter to filterMap callback signature

[Copilot]

The filterMap method callback only received the array element value, inconsistent with standard JavaScript array methods like map and filter which provide both value and index.

Changes

• Maybe.filterMap: Updated signature to fn: (value: U, index: number) => Maybe<V>
• AsyncMaybe.filterMap: Updated signature to fn: (value: U, index: number) => Maybe<V>
• Functional wrapper: Updated all filterMap overloads to include index parameter

Usage

// Filter by position
Maybe.fromNullable(['a', 'b', 'c', 'd'])
  .filterMap((x, i) => i % 2 === 0 ? Maybe.fromNullable(x) : Maybe.fromNullable(null))
  .value(); // ['a', 'c']

// Use index in transformation
AsyncMaybe.fromPromise(Promise.resolve([10, 20, 30]))
  .filterMap((x, i) => Maybe.fromNullable(x + i))
  .value(); // Promise<[10, 21, 32]>

The change is backward compatible—existing code without index parameter continues to work.

Original prompt

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This section details on the original issue you should resolve

<issue_title>filterMap does not provide an index</issue_title>
<issue_description>Currently filterMap has the signature filterMap(value: T): ..., but for an array mapping function it should include the index of the map item, i.e., filterMap(value: T, index: number).</issue_description>

<agent_instructions>Applies to Maybe, AsyncMaybe, and the fp version</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes filterMap does not provide an index #1

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