Support mvindex eval function

[ahkcs]

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Description

The mvindex function returns a subset of a multivalue array using start and optional end index values.
It supports two modes of operation:

1. Single element access:
   mvindex(array, index) — Returns the element at the specified index
2. Range access:
   mvindex(array, start, end) — Returns an array slice from start to end (inclusive)

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Key Features

• 0-based indexing: First element is at index 0 (PPL convention)
• Negative indexing: -1 refers to the last element, -2 to the second-to-last, etc.
• Inclusive end index: Range mvindex(array, 1, 3) returns elements at indices 1, 2, and 3

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Usage Examples

Single Element Access

source=people | eval array = array('a', 'b', 'c', 'd', 'e'), result = mvindex(array, 1) | fields result
# Returns: b

source=people | eval array = array('a', 'b', 'c', 'd', 'e'), result = mvindex(array, -1) | fields result
# Returns: e

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Range Access

source=people | eval array = array(1, 2, 3, 4, 5), result = mvindex(array, 1, 3) | fields result
# Returns: [2, 3, 4]

source=people | eval array = array(1, 2, 3, 4, 5), result = mvindex(array, -3, -1) | fields result
# Returns: [3, 4, 5]

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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[Swiddis]

lgtm

[Swiddis]

Is the args.length < 2 case worth handling?

[ahkcs]

The minimum number of arguments is 2, which is when we use mvindex(<mv>,<start>)

[ykmr1224]

LGTM