Fix argument parsing of flags in the presence of allowPositional=true

[lihaoyi]

Should fix #58 and #60

Previously, we allowed any arg to take positional arguments if allowPositional = true (which is the case for Ammonite and Mill user-defined entrypoints.), even mainargs.Flags. for which being positional doesn't make sense.

    val positionalArgSigs = argSigs
      .filter {
        case x: ArgSig.Simple[_, _] if x.reader.noTokens => false
        case x: ArgSig.Simple[_, _] if x.positional => true
        case x => allowPositional
      }

The relevant code path was rewritten in #62, but the buggy behavior was preserved before and after that change. This wasn't caught in other uses of mainargs.Flag, e.g. for Ammonite/Mill's own flags, because those did not set allowPositional=true

This PR tweaks TokenGrouping.groupArgs to be more discerning about how it selects positional, keyword, and missing arguments:

1. Now, only TokenReader.Simple[_]s with .positional or allowPositional can be positional; Flags, Leftover, etc. cannot

2. Keyword arguments are limited only to Flags and Simple with !a.positional

Added mainargs.IssueTests.issue60 as a regression test, that fails on main and passes on this PR. Existing tests all pass