Improve scaffold command

- Do not fail if a non-JVM platform was selected
- Drop all `scalaOptions`
  - `-Xfuture` has been deprecated in Scala 2.13
  - The other Scala options are not essential for a minimum working
    build file either and can be removed
- If no test frameworks were selected, do not set `testFrameworks`,
  add test modules or create any test folders
- Mark Typelevel Scala as legacy
- Improve formatting of questions
- Reduce output when user provided invalid input
- Test frameworks: Accept "none" as input since it is the indicated
  default value
- When there can be multiple options, allow spaces before/after the
  comma
- Fix crash when selecting Typelevel Scala, pre-releases and several
  test frameworks
- Fix bug which did not allow to specify a custom file path:
  `seed init --build=test.toml`