Add prefix to lines

Puts the same string at the start or end of every line. Blank lines are dropped rather than getting a lonely prefix.

This is the small operation that turns a pasted list into whatever format something else wants: a markdown bullet list, a quoted email reply, a SQL IN clause with quotes and commas, a set of CSS class names. Doing it with find-and-replace on the line break works but breaks on the last line, which has no break after it — that off-by-one is why a dedicated operation is worth having. Combining a prefix and a suffix run is the usual way to build quoted lists: add `'` to both ends, then join with commas.