Disallow mixing tabs and spaces for the sequence block

[clairexen]

I was staring at this

[sequence]
cover r0c1:
  cover r0c2:
    cover r0c3:
      cover r0c4:
        cover r0c5:
          cover r0c6:
            cover r1c1:
              cover r1c2:
                cover r1c3:
		  trace sofar

and didn't understand why I was getting a trace for r0c2 for nearly a whole minute 🤣

instead of just comparing the length of the indent, maybe let's also see if the smaller is actually a prefix of the larger, and call it a syntax error if it doesn't?

[KrystalDelusion]

Rather than comparing the length, the regex will only match it as a child if it starts with the same whitespace, so \t might register as the child of \t, but not . So trace sofar should be a child of cover r0c1 not cover r0c2, and if you were getting a trace for r0c2 then that is a separate bug, but not one I am able to replicate locally (at least not with some quick testing).

36403a8 adds a warning if whitespace is mixed, but I can change that to an error if you think that is better (since it is probably pretty easy to miss the warning).