String concatenation operator?

[gwhitney]

As noted in the (closed) issue #3401:

concat('a', 'b') [for concatenating strings] is a bit cumbersome. Should we introduce a (string and/or array/Matrix) concatenation operator (that would presumably coerce its operands to (strings and/or arrays/Matrices, as appropriate)? If so, what should the operator be? Possibilities that come to mind are:

• ++, if one is willing to require 7 + +5 to have to have the space between the two plus symbols (I certainly would be, because it is super rare for unary + to be useful);
• .+ by very rough analogy with .* and ./ -- it is not elementwise addition, because ordinary matrix addition already is elementwise, but it is the "other sort of addition you might want to do" just like .* is the "other sort of multiplication you might want to do"; or
• && because we are not using this for logical and, so it is open.

There may be other reasonable possibilities.

Jos indicated in the conversation in that issue that mathjs isn't much used for string manipulation, so for the time being concat(a, b) seems fine unless demand for a more convenient notation crops up. So opening this discussion as a place where anyone interested in this (or other) string operator(s) can make their opinions known.