Allow the result of a command to be used in a different command

It would be quite useful to have the possibility to use the result of one command as a value in a different command, similar to how `$(command)` works in unix-like shells. As this syntax is already used for macros, instead, something like `${command}` could be used.

Example:

`data get storage namespace playerdata[${scoreboard players get some_player id}]`