The first in support for modded content, a simple custom block: "rockwool" from the Thermal Expansion and Thermal Foundation mods for Forge:
https://ftb.gamepedia.com/Rockwool_(Thermal_Expansion_3)
This makes use of the Forge handshake (#88#134#144), matching the mod block names from the negotiation to numeric identifiers in the world to steven_blocks. Rockwool was chosen due to ease of implementation, it looks like wool from vanilla (except is fire-proof), and by supporting it the groundwork necessary is laid for more sophisticated mod support.
Tested with Thermal Expansion on 1.7.10, 1.10.2 (FTB Beyond), and 1.12.2 Forge servers.
* Add `modid` macro token, skipped from vanilla mappings
* Add ThermalExpansionRockwool block (1.7.10)
* Register modded blocks by modid->[data], and lookup block metadata
* Save block IDs from ModIdData/RegistryData to World modded_block_ids
* Add namespaced mod ids for ModIdData, \u{1}=block \u{2}=item
* Add ThermalFoundation's Rockwool (1.12.2)