Support splitting up value into array

While a placeholder can specify an array type, the value provided will always just generate an array with a single value. There is no way to split up the value to create an array of values.

We can support this with an additional directive (delimiter) like this:
$[env:foo;type=String[];delimiter=,]
when a value like "a,b,c" is provided, it will be split up into an array of "a", "b", "c"