Improve scaffold command

[tindzk]

• 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 users to specify a custom file path:
  seed init --build=test.toml

If we go through the wizard using the default settings, we get the following result:

Before:

[project]
scalaVersion = "2.13.0"
scalaJsVersion = "0.6.28"
scalaOptions = [
  "-encoding",
  "UTF-8",
  "-unchecked",
  "-deprecation",
  "-Xfuture"
]
testFrameworks = []

[module.example]
root = "shared"
sources = ["shared/src"]
targets = [
  "jvm",
  "js"
]

[module.example.test]
sources = ["shared/test"]

[module.example.js]
root = "js"
sources = ["js/src"]

[module.example.test.js]
sources = ["js/test"]

[module.example.jvm]
root = "jvm"
sources = ["jvm/src"]

[module.example.test.jvm]
sources = ["jvm/test"]

After:

[project]
scalaVersion = "2.13.0"
scalaJsVersion = "0.6.28"

[module.example]
root = "shared"
sources = ["shared/src"]
targets = [
  "jvm",
  "js"
]

[module.example.js]
root = "js"
sources = ["js/src"]

[module.example.jvm]
root = "jvm"
sources = ["jvm/src"]

[tindzk]

@nafg Inspired by #23 (comment), I tried to narrow down the generated configuration to a bare minimum. If there is anything else that could be improved (either in the wizard itself or in the generated build file), please do let me know!