Fix the onboarding flow for the Chrome MV3 build

[kzar]

The onboarding code has some logic to display a welcome banner the
first time the user performs a DuckDuckGo search after installing the
extension. Until now, that code did not work for Chrome MV3 builds,
since it attempted to inject a script into the "main world" by
appending a <script> element - that does not work with MV3 due to the
stricter Content-Security-Policy used. Let's use the
scripting.executeScript API instead to inject the required code into
the "main world".

Note: This code is somewhat convoluted and could likely be simplified
in the future. But for now, with the upcoming MV3 migration
deadline, let's make the minimum changes required to get this
working!

Reviewer: @sballesteros, @sammacbeth

Steps to test this PR:

1. Install the extension
2. Ensure DDG is the default browser if you're running Linux
3. Perform a search from the URL bar
4. Ensure the purple onboarding banner is displayed and a CSP warning isn't shown in the console
5. Perform another search from the URL bar and ensure the onboarding banner is not displayed this time

Note: Test both with MV2 and MV3 builds of the extension!

[sammacbeth]

This worked for me in testing.

I think it would be helpful to file a follow-up to tidying up this code (post MV3 migration):

• Make it a standalone 'component' (to move listeners of our events.js), and keep code in one place.
• Use build flags to only include the component on chrome builds (i.e. BUILD_TARGET !== 'firefox').