Groundwork for JS selfhosted code.

[sagudev]

To enable loading self hosted code in SM, we firstly need to create safe wrappers for InitSelfHostedCode. Secondly we need to expose way to allow loading self hosted JS on JSRuntime creation. Relevant code in mozjs: https://github.com/servo/mozjs/blob/81906696dd9d062692dd5d68bd245cc0306de23e/mozjs/src/rust.rs#L361-L364. And lastly implement this in servo (and hide it behind pref).

We could load pollyfills for yet unimplemented functionalities using self hosted code.

EDIT: Self-hosted code is very restrictive: https://searchfox.org/mozilla-central/source/js/src/vm/SelfHosting.h#31. In Firefox only SM uses self hosted code (so it's only used for ES stuff).

Unresolved questions

Can you load multiple self hosted files or do all need to be merged into one?

How can self hosted JS code benefit servo (find candidates for self hosted code)? What's it's performance impact?

[ektuu]

Hey, I am an outreachy applicant and would wish to work on this issue.
could you assign this to me, or shall I start working and be ready with a draft PR?

Also, if you've any other good first issues, I would be glad, if you could help me identify one.

[sagudev]

This issue isn't really good first issue, especially because it also requires work on mozjs. I suggest looking at E-Easy label for issues.