Allow "empty" single line comments in the spec

[basile-henry]

As I read the grammar in the spec, "//" wouldn't parse as a single-line-comment as it requires as least one non-newline character after the slashes.

[zkat]

oof. Good catch. I'm moving this to the v2 branch, though, because it's a breaking change, and I wanna let others chime in in case anything else like this comes to mind.

[CAD97]

If it wouldn't parse successfully otherwise, it's not breaking, as it's solely extending the allowed grammar. I don't believe there's any case where // is valid currently, where it would potentially be a comment after this.