[Expression]Enable list concatenation for function "concat"

[Danieladu]

concat:

1. If all items are lists, concatenate them with list mode.
   concat([1, 2], [3, 4]) -> [1, 2, 3, 4]

2. If there exists any item that is not a list, use string concatenation mode:
   concat('a', 'b', 1, 2) -> 'ab12'
concat('a', [1, 2]) -> aSystem.Collections.Generic.List 1[System.Object] // unreadable, best to avoid

3. If null exists, skip it
   concat([1, 2], null) -> [1, 2]
concat('a', 1, null) -> 'a1'

issue link: #4014

[chrimc62]

return $"{firstItem?.ToString()}{secondItem?.ToString()}"; [](start = 32, length = 58)

This will be much less efficient on large arrays than the previous implementation since it does not use a string builder. Why not extend ApplySequence to use an accumulator which can use a string builder?

[chrimc62]

🕐

[chrimc62]