Convert `search-index.js` into a plain JSON file.

[Dylan-DPC-zz]

It might help to convert search-index.js into a plain JSON file. I'm pretty sure that JSON.parse is significantly faster than trying to compile and execute JavaScript code.
Originally posted by @ollie27 in #56545 (comment)

Opening this issue to gain traction so that someone can work on it (Ready to mentor if possible)

cc @GuillaumeGomez

This issue has been assigned to @euclio via this comment.

[euclio]

I am willing to work on this.

[Dylan-DPC-zz]

@euclio go ahead and run @rustbot claim.

if you have any issues feel free to ping me here or anywhere else

[euclio]

@rustbot claim

[euclio]

It's easy enough to generate a JSON file instead of a JavaScript file, but how do we want to load the JSON file? I'm not familiar with how the search works: can we request the JSON via XMLHttpRequest and then call addSearchOptions and initSearch asynchronously? We could also embed the JSON in the HTML via a data block, but that would add the cost of the search index to the initial page load.

[Dylan-DPC-zz]

turns out @GuillaumeGomez has some progress on this :/ so maybe you can work on a different issue

[GuillaumeGomez]

Arf sorry @euclio, didn't see this issue so I commented on the other one... Even though I did it, it'd be awesome if you took a look at the PR: #71250 :)

[Alexendoo]

Closed by #71250