:1234: Support for scientific notation and decimal numbers in `si` role

[jan-david-fischbach]

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So far the si role only supported integer numbers. With this PR it becomes possible to use decimal and scientific "e" notation. Additionally a negative sign "-" at the start is also accepted

[agoose77]

@fwkoch I updated this PR as I thought it would be a good place to start looking at roles. Would you be happy to give this another review?

I haven't given much thought to what the renderer should do with this AST yet, e.g. use KaTeX to render the math text.

[agoose77]

@fwkoch thinking of trying to unblock things, is this PR now good to go?

[agoose77]

@rowanc1 we should do something about this. Is there a reason to consider it blocked, or is it mergeable?