[FEATURE] `idx-of` command

[kurtlawrence]

Description

Add a command which uses a predicate and returns the first index where the predicate returns true.

Performs a linear search and stops on the first true of the predicate. Returns the index matched.

Design notes

• The name idx-of could also be find-idx
  • find should be reserved for something which returns a TableRow
  • find-idx would actually align with a future find impl
• What to return on failure?
  • This might be blocked on #156 if something like Option is wanted
  • Alternatively, a numeric like -1 could be returned
• Implement on all the iterables (Table, String)

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Alternatively, a numeric like -1 could be returned

A use case is to find an index for insertion or skipping. In that case, a sensible return value would be beneficial and merits a considered design.