Make optional parts of `LanguageInfo` actually optional!

[DavisVaughan]

I noticed in https://github.com/posit-dev/positron/issues/2098#issuecomment-2384064069 that in a jupyter console we get this pygment error:

See that for language ''? That implies we are actually sending the empty string over for the pygments_lexer part of the language_info part of a kernel_info_reply! https://jupyter-client.readthedocs.io/en/stable/messaging.html#kernel-info

Notice that it says

        # Pygments lexer, for highlighting
        # Only needed if it differs from the 'name' field.
        'pygments_lexer': str,

I'm interpreting this as meaning that its actually an optional field. We were treating it as a required String, but we sent over String::new().

And indeed jupyter_console treats it as an optional field! https://github.com/jupyter/jupyter_console/blob/fddbc42d2e0be85feace1fe783a05e2b569fceae/jupyter_console/ptshell.py#L541-L542

If pygments_lexer is not there, it falls back to name of "R" which is exactly what we wanted all along.

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Now for the cool part. In jupyter console, pygments_lexer is used to look up syntax highlighting using the pygments library. So fixing this actually enables R syntax highlighting in jupyter console for ark!

It uses this, with the key being short name: r https://pygments.org/docs/lexers/#pygments.lexers.r.SLexer