Remove unimplemented validator types for `Parameter`

[nokome]

Currently, the component for Parameter allows the user to select several types of validator:

<select name="validator" class="sc-stencila-parameter-default">
  <option disabled="" class="sc-stencila-parameter-default">Select…</option>
  <option value="ArrayValidator" class="sc-stencila-parameter-default">Array</option>
  <option value="BooleanValidator" class="sc-stencila-parameter-default">Boolean</option>
  <option value="ConstantValidator" class="sc-stencila-parameter-default">Constant</option>
  <option value="EnumValidator" class="sc-stencila-parameter-default">Enum</option>
  <option value="IntegerValidator" class="sc-stencila-parameter-default">Integer</option>
  <option value="NumberValidator" class="sc-stencila-parameter-default">Number</option>
  <option value="StringValidator" class="sc-stencila-parameter-default">String</option>
  <option value="TupleValidator" class="sc-stencila-parameter-default">Tuple</option>
</select>

But many of these are unimplemented (either in the component UI, or in terms of actually setting the value in a kernel). I suggest that we reduce the list to Boolean, Integer, Number, String and Enum (i.e. remove Array, Tuple and Constant).

Enum needs some UI to be able to set the possible options, but that's a separate issue.