Make dummies less dumb

[TheMurderUnicorn]

Summary

Bugfixes "Update Target Dummies to Require Diggable Terrain"

Purpose of change

Target Dummies state in their descriptions they needed to be embedded in the ground AND the construction required a digging tool, however they could be installed anywhere, even on non-diggable terrain.

Describe the solution

Using pre_flags, I made it so the starting terrain MUST be diggable to erect the training dummies. I also updated the furniture (only exists after player constructs it) to state it's embedded via its post in the ground. This also made it so when the construction menu is open, this text is displayed, letting the player know why they can't place it on a road/sidewalk/etc (not that any of us read descriptions)

Describe alternatives you've considered

Adding a second version of both dummies, allowing them to have a weighted base and thus be buildable anywhere. For now, I did not want to do this.

Testing

Verified they could be placed anywhere despite requiring digging. Made changes. Tested that they now only work on diggable terrain and that their descriptions were updated.

Additional context

Closes #73483