Drop langtags on offset text but support on paras

Now that language attributes can be arbitrarily added to any inline,
a special syntax for them doesn’t make much sense. However, that syntax
would be useful on block‐level elements.

There isn’t a real technical reason for the explicit `$` end of the
language tag, since the following white·space could have been used to
end it also, but it makes the syntax a bit more obvious, matches the
existing format with the shebangs, and is less likely to conflict with
id syntaxes (which might include `@` but probably won’t also end in a
`$`).

You can still make id’s like `@es$` if you need by explicitly
language‐tagging: `¶@es$@en$`.

This commit also allows block sigils to not be followed by a space if
they are immediately followed by a pilcrow.