Add theming support.

- Added a number of common themes to our support matrix:
 - AtomOneDark
 - Dracula
 - VS
 - GitHub
 - GoogleCode
 - XCode
 - ... Admittedly these all were randomly picked, we could probably curate these better...
- Added a new `ThemeDialog` UI that can be accessed via `/theme`. It shows your currentlyt available themes and allows you to change them freely. It does **not**:
 - Save the theme between sessions
 - Allow you to hit escape
 - Show a preview prior to selection.
- These themes are from reacts highlight js library.

Fixes https://b.corp.google.com/issues/412797985

packages/cli/src/ui/components/Header.tsx

@@ -7,7 +7,6 @@
 import React from 'react';
 import { Box, Text } from 'ink';
 import Gradient from 'ink-gradient';
-import { Tips } from './Tips.js';
 
 const gradientColors = ['#4796E4', '#847ACE', '#C3677F'];
 
@@ -32,6 +31,5 @@ export const Header: React.FC = () => (
 `}</Text>
       </Gradient>
     </Box>
-    <Tips />
   </>
 );