Fix background HMR by @tyn1998

[octohedron]

As per this comment https://github.com/lxieyang/chrome-extension-boilerplate-react/issues/40#issuecomment-1032384937 I extracted the fix for the HMR, and did some minor updates

• Popup and Greetings are now tsx files
• Added a button to the Popup with a counter as an example
• Added an example for listening to tab changes in the background

chrome.tabs.onUpdated.addListener(function (tabId, changeInfo, tab) {
  if (changeInfo.status == 'complete') {
    console.log('Tab finished loading', tabId, tab);
    // do your things
  }
});

• Updated the webpack config and README

TODO: Add instructions as explained in https://github.com/lxieyang/chrome-extension-boilerplate-react/issues/40#issuecomment-1032384937

[amehito]

expecting for merger

[moshOntong-IT]

it seems the package author is not active

[VecHK]

good job

[joshsmith]

@lxieyang I know things come up (I've been there an OSS maintainer in the past). If you're not able to remain active, would you mind considering opening up shared responsibility to contributors who appear interested in helping to maintain the repo?