Merge branch 'trunk' of https://github.com/WordPress/performance into add/mark-url-metrics-stale-on-tag-visitor-change

* 'trunk' of https://github.com/WordPress/performance:
  Update README with new lazy bgimage feature
  Remove logic made obsolete by #1658
  Add missing tests for lazy-load script/style functions
  Rename previous lazy-load code now to be for video specifically
  Combine script and style addition
  Move image_prioritizer_get_lazy_load_script to helper.php
  Add test cases for lazy loading background images
  Remove unused variable in test case
  Improve formatting of inline style tag
  Fix typo in comment for test case
  Update .gitignore to ignore minified CSS files
  Update Webpack config to minify CSS files
  Move inline CSS to separate file
  Simplify conditional check for viewport
  Avoid relying on `has-background` class for lazy-loading background images
  Lazy load bg images only when URL metrics for both mobile and desktop are available
  Refactor lazy-loading styles to inline implementation
  Lazy load background images
  Disable loading of bg images not in initial viewport

Author: westonruter

.gitignore

@@ -25,6 +25,7 @@ nbproject/
 build
 .wp-env.override.json
 *.min.js
+*.min.css
 *.asset.php
 
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