Add: consist_current_power in varact2 for trains

[WolfieFox]

OpenTTD has had these vars exposed since r3594 (Feb 2006) so I think we should be good to go on using them

[PeterN]

formation isn't a word that's used anywhere else. The normal term that's already used is consist.

[WolfieFox]

Valid, renamed the var to match.

[2TallTyler]

Needs updating to squash the fixup commit, and rebasing so the Black CI check passes.

[2TallTyler]

Commit name doesn't match the format (but the PR title is correct). 🙂

[WolfieFox]

Commit name doesn't match the format (but the PR title is correct). 🙂

Damnit, the rebase command prompted me three times and I didn't even parse it the second time. One moment...

[2TallTyler]

This variable is not published in NewGRF specs, but I found it. 🙂

For others' reference: the source code exposes it in vehicle specific properties -> VEH_TRAIN -> case 0x74 (offset by 0x80):
https://github.com/OpenTTD/OpenTTD/blob/master/src/newgrf_engine.cpp#L924

If you're willing, this could use adding to the specs.
https://newgrf-specs.tt-wiki.net/wiki/VariationalAction2/Vehicles

If/when this is merged, it will also need to be added to the NML specs.
https://newgrf-specs.tt-wiki.net/wiki/NML:Vehicles