Clean up BibTeX parsing.

Previously there was a messy code path that gave strange
results in some cases, not passing through raw tex but
trying to extract a string content.  This was an artefact
of trying to handle some special bibtex-specific commands
in the BibTeX reader. Now we just handle these in the
LaTeX reader and simplify parsing in the BibTeX reader.
This does mean that more raw tex will be passed through
(and currently this is not sensitive to the `raw_tex`
extension; this should be fixed).

Closes #7049.

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