Display multi-line string results correctly

[mkroetzsch]

Strings that contain linebreaks are shown in result tables with escape sequences when using the web app. Example program:

test("""A longer
text  with mutliple
    lines.
""") .

@output test.

Output in table:

"A longer\ntext with mutliple\n lines.\n"

This example also shows that whitespace is stripped where it should be part of the value. During reasoning, the whitespace is correctly processed (strings differing only in whitespace are distinct lines in results), but their difference is no longer visible in the web app's result table.