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.test.ts

@@ -51,7 +51,7 @@ describe('SelectionManager', () => {
   });
 
   function stringToRow(text: string): LineData {
-    let result: LineData = [];
+    const result: LineData = [];
     for (let i = 0; i < text.length; i++) {
       result.push([0, text.charAt(i), 1, text.charCodeAt(i)]);
     }