Add a gamerule to display a message when a respawn point is invalidated

Perceived Problem: New players can be unaware that breaking your bed removes your spawn point and will send you all the way back to worldspawn. Similarly, players on servers have no warning if somebody else removes their bed. And sometimes we just have little moments.

Proposed Solution: When the bed or respawn anchor that is currently set as your spawn point is broken, a small message should appear (like the "you cannot sleep right now, there are monsters nearby" popup) on the player's screen saying "you no longer have a valid spawn point" or similar. This way they will have an extra prompt to avoid problems upon death.  The rationale behind making this a gamerule change is that some servers (particularly anarchy ones) may desire to keep the potential for pranks/traps involving the invalidation of a spawn point.  If it is a gamerule, then there remains the option to keep the message hidden.

This change does represent a difference in the way the game handles beds - instead of only checking if a bed is present at a player's spawn point after they click respawn, it would now have to check if a player's spawnpoint is tied to a respawn block whenever that block is broken.  This may increase computer load, but I lack the technical expertise to comment on whether that increase would be noticeable or not.