[Request] Support plain text in chemical formulas

[github-young]

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1. For some chemicals with plain text in the formula (subscript/superscript, and hyphen), it is difficult or impossible to typeset with typsium.
2. The font in chemical formula representing non-numerical meanings should be upright. It is default in LaTeX, but seems that they are italic in Typst.
   For example:

• The mixture of CuO and $Cu_2O$ should be $Cu_xO$:

#ce("Cu_x O")

• The hyohen in "He-Ne" (referring to a kind of laser):

#ce("He-Ne")

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Probably use the mhchem convention, where a pair of {} is used for these plain text in the chemical formula:

% \usepackage[version=4]{mhchem}
\ce{Cu_{x}O}

corresponds to:

#ce("Cu_(x)O")

but I don't know if the brackets are legal here in Typst.