Introduce line marker for wasm_bindgen attributes in WebAssembly projects

[avrong]

Expected workflow: navigating from Rust statement to JS one. Then having the ability to navigate to JS parts related to it.

Current implementation

How it works

Related to #3066

[Undin]

Suggestion: use more lightweight gutter icon
The current one looks too heavy (in my opinion, of course)
For example, C++ similar icon

Also, it would be great to come up with an extendable template for such icons because we can provide a similar feature for extern functions
Of course, don't insist to do it in this PR

[avrong]

There is an option to show JS file icon like PyCharm does for Flask templates. But I'm not sure if it is convenient to use a file icon here.

We can use something similar to the C++ icon, then I think we should make it distinctive (but not standing out) from others as wasm_bindgen is a third-party thing to the language itself and its IDE features.

A similar feature for extern functions would be cool. I'm not aware of the details but will try to find a way to keep these features in one area for the future.

[Undin]

@avrong personally, I like an approach with file icon since it doesn't require any additional icons, quite extensible and it's already used in other JB products. But we should use TS icon instead of JS in this PR, shouldn't we?
@ortem what do you think?

[artemmukhin]

@Undin I think JS/TS icons is a clear and simple solution 👍

[artemmukhin]

bors r+

[Undin]

What about tests?

[bors]

Build succeeded:

• check

[artemmukhin]

@Undin We are going to add them later, in a separate PR