feat: add experimental support for soft navigations

[vegerot]

Summary:

Recent versions of Chromium have added support for tracking web vitals metrics across soft navigations. This change adds support for tracking these metrics in the extension.

TODO:

• [ ] document the new reportSoftNavs option in the README web-vitals package
• [ ] Hide the option in the options page if one of the following is false:
  1. The browser supports the soft-navigation entry type
  2. The web-vitals package has been built with the soft-navs branch

[vegerot]

The easiest way to build the extension is just to run npm install instead of npm run build

[tunetheweb]

Thank you for the work on this. And I understand why this would be useful, but while soft-nav support is experimental (and particularly at the moment when it's not being run in the origin trial while we're making changes to the implementation), I'd prefer not to merge this change and include soft nav support in the official extension (even if it's marked as experimental and disabled in the cases you describe above).

We can reevaluate this when a new origin trial is launched and this becomes more generally available.

Happy to keep this branch open in the meantime so people can install this build if they wish, but just wanted you to make you aware why we're unlikely to accept this PR for now.

[vegerot]

Thanks for taking a look. That makes sense. I'm working on this so my team can use it. They're already using this PR to grab the code from, so this works for them too 🙂