Prefer const vs let for variables that are only declared once

Requires that variable declarations use `const` instead of `let` and `var` if possible.

This is mostly for code readability, performance wins are debatable.

src/addons/attach/attach.ts

@@ -51,7 +51,7 @@ export function attach(term: Terminal, socket: WebSocket, bidirectional: boolean
         str = myTextDecoder.decode(ev.data);
         displayData(str);
       } else {
-        let fileReader = new FileReader();
+        const fileReader = new FileReader();
 
         fileReader.addEventListener('load', () => {
           str = myTextDecoder.decode(this.result);