We previously supported (and primarily used) a non-standard text format for conditionals in which the condition, if-true expression, and if-false expression were all simply s-expression children of the `if` expression. The standard text format, however, requires the use of `then` and `else` forms to introduce the if-true and if-false arms of the conditional. Update the legacy text parser to require the standard format and update all tests to match. Update the printer to print the standard format as well. The .wast and .wat test inputs were mechanically updated with this script: https://gist.github.com/tlively/85ae7f01f92f772241ec994c840ccbb1