Set eslint for our hand-written js files

[inkydragon]

eslint could

• find some errors
• format the code

[inkydragon]

error example

Documenter.jl/assets/html/js/copy.js

Lines 36 to 52 in e5441ba

	return new Promise(function (resolve, reject) {
	try {
	const el = document.createElement("textarea");
	el.textContent = text;
	el.style.position = "fixed";
	el.style.opacity = 0;
	document.body.appendChild(el);
	el.select();
	document.execCommand("copy");
	resolve();
	} catch (err) {
	reject(err);
	} finally {
	document.body.removeChild(el);
	}
	});

Line 50
'el' is not defined.eslint (no-undef)

fix patch

     return new Promise(function (resolve, reject) {
+      const el = document.createElement("textarea");
       try {
-        const el = document.createElement("textarea");
         el.textContent = text;

[mortenpi]

Yeah, this seems like a good idea to me, together with a CI pass to check PRs.