feat: Implement relative positioning for commands

[Nul-led]

Why:

Closes issue link Allows more accurate positioning of summon and teleport

Summarize what's being changed (include any screenshots, code, or other media if available):

Uses relative entity position of player by replacing "~" in the command args with the player position Examples:

• teleport xArg = "~", yArg = "~100" will add 100 units to the players y position
• summon defender at "~ ~-1000" will summon a defender at the player's x coordinate but offsets the y coordinate by -1000

Confirm the following:

• [x] I have tested these changes (by compiling, running, and playing) and have seen no unintended differences in gameplay