Correctly migrate leading-* classes

[RobinMalfait]

This PR fixes the upgrade tool by properly migrating the leading-[<number>] classes.

The issue is that leading-[<number>] maps to the number directly, but if you use a bare value, then it's a multiplier for based on the --spacing value.

E.g.:

leading-[2]:

.leading-\[2\] {
  --tw-leading: 2;
  line-height: 2;
}
@property --tw-leading {
  syntax: "*";
  inherits: false;
}

leading-2:

.leading-2 {
  --tw-leading: calc(var(--spacing) * 2);
  line-height: calc(var(--spacing) * 2);
}
@property --tw-leading {
  syntax: "*";
  inherits: false;
}

This PR will now prevent migrating arbitrary values to bare values for leading-* utilities.

That said, this does introduce a small improvement where leading-[1] is migrated to leading-none.

Fixes: #15924