Support if (cond) do
Entirely by accident, Pico-8 supports an illegal variant of `if (cond) then\n ... end` that looks like `if (cond) do\n ... end`. This is a result of Pico-8's "short-if" feature accidentally accepting the multiline do block as if it were on a single line. If it's supported, people will use it (intentionally or otherwise), e.g. 16314, 16350, 16992, 16999, 17135, 17238, 17406, 17550.

Pico-8 is unlikely to remove support for this accident, so picotool should support it. Whether it needs to _preserve_ it, I don't know. It's possible authors of old carts used this trick to reduce character count, though hopefully the relatively new emphasis on token count makes this obsolete. To preserve it, we'd need to extend the grammar. It might be easier to convert "do" to "then" and risk pushing carts over the limit, or it might not.
