Add mapsto (Proposal 24)

[T0mstone]

This PR implements Proposal 24 (iteration 2) from the Symbol Proposals Document.

I also added to as per our Discussion on Discord.

This is a draft since it would be better done with the tools from #27.

[MDLC01]

Why not also adding to.long?

[T0mstone]

That wouldn't work when converting them to #27's aliases, unless you also want to add all other modifiers of arrow.r to to. It seems like LaTeX doesn't have \longto either.

I could of course extend the alias system with a "copy some of the variants, but not all" declaration, but you previously called that "overkill".

There's also still the option of not using aliases for these, but I feel like we should avoid duplication if possible.

[T0mstone]

Maybe the better solution is getting rid of to again. As you already said, typst already has ->. The name "to" also isn't nearly as semantic as "mapsto", so it doesn't really add much.

[MDLC01]

There's also still the option of not using aliases for these, but I feel like we should avoid duplication if possible.

I don't think the way we decide to implement a symbol (using aliases or not), or the way we implement our alias system, should dictate what we decide to add.

I don't know if we should add to, or to.long, but we should not take into consideration the way our alias system works when adding them. The final product should be good for the end users. Having good names is a higher priority than not duplicating some characters.

Just to be clear, I'm not saying that we should or shouldn't add to or to.long, just that this decision should be independent of the alias system.

The name "to" also isn't nearly as semantic as "mapsto", so it doesn't really add much.

This is a good point.

[T0mstone]

Given that you agreed to my last point, I've removed to again now.