Improved font style export

New feature

• new values added to font property (https://github.com/lukasoppermann/design-tokens#figma-style-design-tokens)
  • fontWeight – numeric value for font weight between 100 and 900
  • fontStyle – italic, oblique or normal
  • fontStretch – normal, condensed or expanded
  • fontStyleOld – original value from figma's fontStyle

Breaking change

• the old fontStyle value was replaced with the new one (one of the three values italic, oblique or normal)

This fixes #55 by introducing a lookup table and splitting font style into weight, style and stretch