StringFormatter

General purpose string formatter, with the following features:

1. Invokes the show extension method on the interpolated arguments.
2. Sequences can be formatted using the desired separator between two % signs, eg i"myList = (${myList}%, %)"
3. Safe handling of multi-line margins. Left margins are stripped on the parts of the string context before inserting the arguments. That way, we guard against accidentally treating an interpolated value as a margin.