feat(poster): add support for marking player as maincontent

[kontrollanten]

Description

When maincontent is set to true the poster img element will get loading=eager and fetchpriority=high to decrease LCP time.

Usage

maincontent: true

    var player = videojs(vid, { maincontent: true });

Default mode

    var player = videojs(vid);

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
  • Has no DOM changes which impact accessiblilty or trigger warnings (e.g. Chrome issues tab)
  • Has no changes to JSDoc which cause npm run docs:api to error
• Reviewed by Two Core Contributors

I'm not sure if any example should be created, and if so where to put it. The same goes with the documentation.

closes #9171

[welcome]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.94%. Comparing base (69ea0e0) to head (edd7b45).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9173      +/-   ##
==========================================
+ Coverage   84.32%   84.94%   +0.62%     
==========================================
  Files         120      120              
  Lines        8153     8157       +4     
  Branches     1964     1967       +3     
==========================================
+ Hits         6875     6929      +54     
+ Misses       1278     1228      -50     

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[mister-ben]

At the time loading="lazy" was added, Lighthouse would complain if it were absent, even on a prominent player. Iirc it was said that browsers would disregard the lazy directive on elements it deemed to have priority. Since the recommendation has changed, this change is more than reasonable.

Nit: The option name should be camel case like other options.

[kontrollanten]

Thanks for reviewing! I've now changed maincontent to mainContent.