transcripts: don’t require tags to be in specific order

[sellout]

A transcript like

```unison :error :hide
this is a parse failure, right?
```

isn’t valid because the info string parser expects :hide to come before :error.

I don’t think the parser should care which order the tags appear in.

However, in the case of unison blocks in particular, there is an optional scratch file name at the end. It isn’t marked by a tag. I think this probably should still have to be last.