docstr: Add conditional substitutions to a Variable Font.
  
  See `fontTools.varLib.featureVars.addFeatureVariations`.
code: def addFeatureVariations(
      self, conditionalSubstitutions, featureTag="rvrn"
  ):
      """Add conditional substitutions to a Variable Font.
  
      See `fontTools.varLib.featureVars.addFeatureVariations`.
      """
      from .varLib import featureVars
  
      if "fvar" not in self.font:
          raise KeyError(
              "'fvar' table is missing; can't add FeatureVariations."
          )
  
      featureVars.addFeatureVariations(
          self.font, conditionalSubstitutions, featureTag=featureTag
      )
  
imports: from .ttLib import TTFont, newTable
  from .ttLib.tables._c_m_a_p import cmap_classes
  from .ttLib.tables._g_l_y_f import flagCubic
  from .ttLib.tables.O_S_2f_2 import Panose
  from .misc.timeTools import timestampNow
  import struct
  from collections import OrderedDict
label_desc: Use image node_id 29 for calling the FontBuilder obj's underlying member method code with example usage: obj.addFeatureVariations(conditionalSubstitutions, featureTag) without return types