build: use multiline string literals when necessary

Update the output and hint fields in the `exercises` slice to use a
multiline string literal when the string have multiple lines or use the
`"` character.  This will greatly improve readability.
Additionally, remove the trailing whitespace on each line and check it
in the validate_exercises function.

Update the output comparison logic in ZiglingStep, since the current
code assumes that the string has only one line.

Update test/tests.zig to use the new `CheckNamedStep` in test case 1,
since RunStep.StdIo.Check is no longer able to correctly check the
output.

Fixes #283

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