improve: add callback as an optional argument to handle eval_js result

[shiqimei]

This PR tries to provide an optional argument callback_name to the function firenvim#eval_js.

The new signature of this function will be like the following:

firenvim#eval_js(code: string, callback_name?: string)

In this way, we can handle the result of the javascript evaluation in the vim script.

Usages

function EvalJSCallback(result)
    echom 'result' a:result
endfunction

function EvalJS()
    let js = "location.href"
    call firenvim#eval_js(js, 'EvalJSCallback')
endfunction

output: result "https://www.google.com"

function EvalJSCallback(result)
    echom 'result' a:result
endfunction

function EvalJS()
    let js = "new Promise(resolve => fetch('https://jsonplaceholder.typicode.com/todos/1').then(response => response.json()).then(resolve))"
    call firenvim#eval_js(js, 'EvalJSCallback')
endfunction

output: result {"userId":1,"id":1,"title":"delectus aut autem","completed":false}

[glacambre]

@lolimay Thanks a lot for taking the time to implement this feature! It unearthed a couple of bugs elsewhere which are now fixed :).