Make planck more useful (Proposal 26)

[MDLC01]

Description

This PR implements Proposal 26 (iteration 1) from the Symbol Proposals document. It changes planck to ħ (which gets italicized to ℏ in math thanks to typst/typst#5492), and removes the planck.reduce variant. For more context, see the discussion on Discord.

There is no way to take advantage of the symbol deprecation mechanism introduced in #19 to make this change smoother.

Breaking changes (cc. @laurmaedje)

planck.reduce becomes invalid, and planck changes codepoint.

[T0mstone]

I don't like how this changes people's documents without warning.

I'm still a bit conflicted on whether that's just a necessary evil or whether we should look into adding some kind of ad-hoc warning in typst before making this change here...

[MDLC01]

I guess we could split this into multiple phases if we really want to make sure it does not break people's documents:

• First, mark planck (but not planck.reduce) as deprecated (people should use a regular h instead).
• In the next version, change planck to be the version with the bar, and deprecate planck.reduce (bare planck should be used instead).
• Finally, in a third version, remove planck.reduce.

I'm not sure it would be worth spanning this over three versions, but if it breaks many documents, I think this is the only way to make a progressive change.

[T0mstone]

Hmm..... It may be a bit dumb, but that's probably the most robust way to go about it.

The more I think about it, the more I actually like the idea.

@laurmaedje would you say this is warranted or is it overkill?
I can also accept the viewpoint that doing this change in one go is just something users will have to deal with (tho I don't really like it).

[laurmaedje]

Very similar to what I proposed here for read: typst/typst#5672

For read, it's a bit more critical than for planck, but I could imagine going about it that way here as well.

[MDLC01]

All this would have to be done on Typst's side, tough. Codex does not have the ability to express such precise warnings.

[T0mstone]

After another brief discussion of this on discord, I'll drop my concern for this, so LGTM.

[MDLC01]

To be clear, what the next Typst changelog should mention is that planck.reduce was renamed to simply planck, and previous uses of planck can be replaced with simply h in math mode.