Tolerate malformed comments in shebang lookahead

Our `SHEBANG` rule did not accept `#!/*`, even though that's a valid
shebang per `rustc`.  The trouble is the interaction between the
negative lookahead and the cut within `BLOCK_COMMENT`: the cut forces
an immediate error even within the context of a negative lookahead.
We rely on that semantic elsewhere.

To fix this, let's add a `SHEBANG_BLOCK_COMMENT` rule that accepts
what `rustc` does.