Only split doctest lang strings on `,`, ` `, and `\t`. Additionally, to
preserve backwards compatibility with pandoc-style langstrings, strip a
surrounding `{}`, and remove leading `.`s from each token.
Prior to this change, doctest lang strings were split on all
non-alphanumeric characters except `-` or `_`, which limited future
extensions to doctest lang string tokens, for example using `=` for
key-value tokens.
This is a breaking change, although it is not expected to be disruptive,
because lang strings using separators other than `,` and ` ` are not
very common
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