Add toText and writeValues on Position and Box

[navispatial]

Planned methods for Position:

  /// A string representation of coordinate values separated by [delimiter].
  ///
  /// Use [decimals] to set a number of decimals (not applied if no decimals).
  ///
  /// A sample with default parameters (for a 3D position):
  /// `10.1,20.3,30.3`
  ///
  /// To get WKT compatible text, set `delimiter` to ` `:
  /// `10.1 20.2 30.3`
  String toText({
    String delimiter = ',',
    int? decimals,
  });

  /// Writes coordinate values of [position] to [buffer] separated by
  /// [delimiter].
  ///
  /// Use [decimals] to set a number of decimals (not applied if no decimals).
  ///
  /// A sample with default parameters (for a 3D point):
  /// `10.1,20.3,30.3`
  ///
  /// To get WKT compatible text, set `delimiter` to ` `:
  /// `10.1 20.2 30.3`
  static void writeValues(
    Position position,
    StringSink buffer, {
    String delimiter = ',',
    int? decimals,
  });

Similar also for Box.

These were originally defined on the geocore package as valuesAsString and writeValues, but the first one is renamed to be more consistent with naming conventions of the geobase package.

[navispatial]

Implemented in geobase 0.4.0-dev.1.