feat(/convert): per-type formulas

[kyranet]

Some units support multiple targets, however, their formulas to the SI differ.

For example:

• 1 eV equals to:
  • 1.6E-16 g
  • 1.6022E-19 J

The per-type formula used would be of the opposite unit's type, whichever matches, meaning that:

• from:eV to:g should return 1.6E-16 g
• from:eV to:J should return 1.6022E-19 J