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/SelectionManager.ts

@@ -179,7 +179,7 @@ export class SelectionManager extends EventEmitter implements ISelectionManager
 
     // Get first row
     const startRowEndCol = start[1] === end[1] ? end[0] : null;
-    let result: string[] = [];
+    const result: string[] = [];
     result.push(this._buffer.translateBufferLineToString(start[1], true, start[0], startRowEndCol));
 
     // Get middle rows
@@ -586,7 +586,7 @@ export class SelectionManager extends EventEmitter implements ISelectionManager
    * @param event The mouseup event.
    */
   private _onMouseUp(event: MouseEvent): void {
-    let timeElapsed = event.timeStamp - this._mouseDownTimeStamp;
+    const timeElapsed = event.timeStamp - this._mouseDownTimeStamp;
 
     this._removeMouseDownListeners();