Allow for more complicated conditions

[moxvallix]

Currently we only have the basic if syntax for doing conditions. We want to be able to have more control over the underlying execute command.

One idea is maybe to have something like:

if `entity @s[tag=foo]` {
	...
}

Another idea is to add trailing lambda syntax for compile time functions, allowing for the standard library to provide:

&execute(`if entity @s[tag=foo]`) {
	...
}

[moxvallix]

These two could also be nicely nested:

&execute(`as @e[tag=example.entity]`) {
	if `predicate example:in_water` {
		...
	} else {
		...
	}
}