Add array method `includes`

[Shadow-Devil]

Description

I'm quite surprised that there currently is no array.contains(item) method. Of course, I could use a query expression or array.indexOf(item) !is () or array.some(a => a == item) but I think it would be convenient to have this simple method. Another idea is to use a similar syntax to Python (e.g. item in array) since in is already a keyword.

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-> Standard Library

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[MaryamZi]

Moving this to the spec repo since this is a suggestion for the array langlib.

[jclark]

If we add it, it should be called includes for consistency with lang.string.

[jclark]

ECMAScript has this too: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/includes

[jclark]

I think probably we should add for consistency with lang.string and ECMAScript/JavaScript. It should have an additional argument, defaulting to zero, saying where to start looking from.

[jclark]

# Tests whether an array has a member equal to a value.
#
# + arr - the array in which to search
# + val - member to search for
# + startIndex - index to start the search from
# + return - `true` if there is a member of `arr` equal to `val` at an index >= `startIndex`,
#    or `false` otherwise
public isolated function indexOf(AnydataType[] arr, AnydataType val, int startIndex = 0) returns int? = external;