Consistently use .slant

[Enivex]

Take two.

• Add the most closely related additional variants. There are a bunch more, but those can wait to avoid more bikeshedding.
• This should unblock Precedes and succeeds under relation (Proposal 10) #36
• We should aim to have a way for users to consistently use the slanted variants by default in the future, as these aren't very ergonomic.

[MDLC01]

Shouldn't we wait before we have a final solution to the .slant vs. .alt vs. .seq problem before adding all those?

[Enivex]

Shouldn't we wait before we have a final solution to the .slant vs. .alt vs. .seq problem before adding all those?

I answered this on Discord, but I think it's valuable to have it here. It seemed that getting consensus about the seq solution was pretty much impossible.

My thinking is that we go for this now, but try to find a more ergonomic way for those who prefer the slanted variants to use those by default in the future.

By the way: We should probably also discuss what to do about parallel.slanted.* and div.slanted.*. Those are slightly different, in that it's the symbol itself that is slanted, and not the modifier. Not sure that's a big deal though.

[MDLC01]

For reference, this PR supersedes #89.

.slant vs. .slanted is indeed an inconsistency, but I agree we should merge this for now. The inconsistency already exists anyway, and this will allow us to move forward with #36 as you mentioned.

[knuesel]

There are a bunch more, but those can wait to avoid more bikeshedding.

Would you have a list at hand? I think this PR introduces good, consistent naming but it would be nice to have a sense of how it would fit with those other symbols.

[Enivex]

You can check @MDLC01 's proposal document. But I purposely do not want to get into naming those here.

[MDLC01]

The document in question.

[T0mstone]

The changelog needs to be updated now too (cf. #121 (review)).

[MDLC01]

The following should be a complete list of slanted forms that aren't yet in Codex:

⪕
⪖
⫹
⫺
⪛
⪜
≲\vs{1}
≳\vs{1}
⪝\vs{1}
⪞\vs{1}
⪬\vs{1}
⪭\vs{1}
⋚\vs{1}
⋛\vs{1}

I excluded symbols that only exist in a slanted form.

In order to move forward with this PR, I think the .dot variants should be excluded from it because there are other variants with single decorative dots (see image below) that should be discussed separately.