Vanilla terrain_textures.json keys not recognized in blocks.json

[HIPPOKLEIDES]

Keys for textures in the vanilla terrain_textures.json aren't being recognized. A lot of errors could be cleared out if they were taken into account.

While there has been talk of blocks.json being removed in place of something else (separate files?), the keys in terrain_textures.json will still need to be referenced in some place.

[Xterionix]

@DaanV2 This'd fall more closely under the vanilla-data repo, correct?

[DaanV2]

Should be indeed 🤔