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/renderer/ColorManager.test.ts

@@ -33,7 +33,7 @@ describe('ColorManager', () => {
 
   describe('constructor', () => {
     it('should fill all colors with values', () => {
-      for (let key of Object.keys(cm.colors)) {
+      for (const key of Object.keys(cm.colors)) {
         if (key !== 'ansi') {
           // A #rrggbb or rgba(...)
           assert.ok(cm.colors[key].css.length >= 7);

src/renderer/ColorManager.ts

@@ -42,7 +42,7 @@ export const DEFAULT_ANSI_COLORS = (() => {
 
   // Fill in the remaining 240 ANSI colors.
   // Generate colors (16-231)
-  let v = [0x00, 0x5f, 0x87, 0xaf, 0xd7, 0xff];
+  const v = [0x00, 0x5f, 0x87, 0xaf, 0xd7, 0xff];
   for (let i = 0; i < 216; i++) {
     const r = v[(i / 36) % 6 | 0];
     const g = v[(i / 6) % 6 | 0];
@@ -75,7 +75,7 @@ function fromHex(css: string): IColor {
 }
 
 function toPaddedHex(c: number): string {
-  let s = c.toString(16);
+  const s = c.toString(16);
   return s.length < 2 ? '0' + s : s;
 }