Convert to PreTeXt features

[oscarlevin]

Add a "convert" command that takes the current editor window, or currently selected text, and converts it to valid PreTeXt. This will not be useful for converting an entire document, but can be helpful if an author pastes in text from a different document, such as LaTeX or markdown.

Specifically, the function should:

• Guess what format each paragraph of the input is in
• Replace standard markup from that format with reasonable guesses for equivalent markup in PreTeXt
• Highlight the changes for review (possibly).

A good first step would be to search for pairs of dollar signs around text (indicating math mode) and replace them with <m>...</m>. But note that this should not happen if the dollar signs are inside a \begin{tikzpicture}...\end{tikzpicture} pair.