[FEATURE_REQUEST] Null coalescing assignment

[Nico314159]

Description

In JavaScript, the x ??= y operator will assign x the value of y only if x is null. This is highly useful for setting default values of variables. To Minecraft-ify this, I propose that the syntax be:

$<variable> ??= <integer>;
$<variable> ??= $<variable>;

which compiles down to

execute unless score <foo> <objective> = <foo> <objective> run scoreboard players set <foo> <objective> <integer>;
execute unless score <foo> <objective> = <foo> <objective> run scoreboard players set <foo> <objective> = <bar> <objective>;

This would be useful for the same reason as in the original JavaScript, as initializing scoreboard variables is a common need.

[Nico314159]

Already set up a GitHub branch for this one too: https://github.com/Nico314159/jmc/tree/null-coalescing