Implement a `JSON::erase_if` method to erase array items that match a predicate

[jviotti]

I imagine something like this:

  /// This method deletes a set of array elements given a predicate. For
  /// example:
  ///
  /// ```cpp
  /// #include <sourcemeta/core/json.h>
  /// #include <cassert>
  /// #include <iterator>
  ///
  /// sourcemeta::core::JSON array =
  ///   sourcemeta::core::parse_json("[ 1, 2, 3 ]");
  /// array.erase_if(array,
  ///   [](const auto &item) { return item.to_integer() % 2 == 0; });
  /// assert(array.size(), 2);
  /// assert(array.at(0), 1);
  /// assert(array.at(1), 3);
  /// ```
  auto erase_if(const std::function<bool(const JSON &)> &predicate) -> void;

It could internally just use std::erase_if as arrays are std::vectors