Move format-version constant to rustdoc-json-types

[jyn514]

Right now, the format-version is in rustdoc and not in rustdoc-json-types, which has two downsides:

1. It's harder to make sure they're kept in sync
2. Downstream users can't know the version ahead of time, they're forced to hard-code it in their own code as well.

We should move it to rustdoc-json-types instead.

Originally posted by @jyn514 in #88604 (comment)

[0xPoe]

@rustbot claim

[jyn514]

Hi @hi-rustin, are you still planning to work on this?

[0xPoe]

Hi @hi-rustin, are you still planning to work on this?

Sorry, I've been a little busy lately. I will cancel the assignment. Let someone who wants to fix it.

[yuvaldolev]

Hi @jyn514 !
Can I claim this issue if no one else is working on it?

[jyn514]

@yuvaldolev go for it :)

[yuvaldolev]

@rustbot claim

[yuvaldolev]

Hi, @jyn514
What, in your opinion, is the best place for the format-version constant within rustdoc-json-types?
I'm currently thinking of placing it in the lib.rs file, at the crate root - does that sound good?

[jyn514]

@yuvaldolev yup, that sounds fine :)