New check: Check METADATA.pb category

[m4rc1e]

Just pushed an update to Encode whose category was "SAN_SERIF" when it should've been "SANS_SERIF".

google/fonts#2515

Check should ensure that the METADATA.pb category is one of the following ["MONOSPACE", "SANS_SERIF", "SERIF", "DISPLAY", "HANDWRITING"]

[felipesanches]

I'll start with this as a code-test to describe (and enforce) the expected behavior for the proposed new check: