Build/Test Tools: Introduce local visual regression testing.
Adds the ability to locally run visual regression testing for wp-admin pages via npm run test:visual. Snapshots are stored on contributors' local machines.
Note:
Wiring to the CI is not included. Why? The challenges for the CI are storage of the artifacts and unreliability of testing these across different environments.
This commit is a first step towards visual regression testing. Running it locally provides a learning opportunity which could help to craft how to build it into the automated CI process.
Props isabel_brison, andraganescu, azaozz, danfarrow, desrosj, hellofromTonya, justinahinon, netweb, talldanwp.
Fixes #49606.