Add Heat Router block

[l-Luna]

Implements suggestion #3760.

Heat Routers split heat into three directions. They accept heat from any direction, but a redirector heating its sides doesn't work due to heat cycles (just like a circle of redirectors).

Currently, this reuses the redirector sprites, and has the same icon, since heat outputs don't appear on icons.

---

If your pull request is not translation or serverlist-related, read the list of requirements below and check each box:

• [x] I have read the contribution guidelines.
• [x] I have ensured that my code compiles, if applicable.
• [x] I have ensured that any new features in this PR function correctly in-game, if applicable.