The `trim()` function removes specific leading and trailing characters from
a given input string. If the characters to trim are unspecified, the space, tab,
carriage return and newline characters will be used by default.

Returns a copy of the input string with the specified leading and trailing
characters removed.

Returns `null` if the given input argment is not a valid string value.

-- Testcase --
{%
	printf("%.J\n", [
		// not specifying trim characters will trim whitespace
		trim(" 	Hello World!	\r\n"),

		// if trim characters are specified, only those are removed
		trim("|* Foo Bar +|", "+*|"),

		// trim does not affect characters in the middle of the string
		trim("  Foo  Bar  "),
		trim("|Foo|Bar|", "|")
	]);
%}
-- End --

-- Expect stdout --
[
	"Hello World!",
	" Foo Bar ",
	"Foo  Bar",
	"Foo|Bar"
]
-- End --


Supplying an invalid string will yield `null`.

-- Testcase --
{%
	printf("%.J\n", trim(true));
%}
-- End --

-- Expect stdout --
null
-- End --