Fix Incorrect Text Domains in Translatable Strings

[ravigadhiya007]

While reviewing plugin translations, I noticed that some translatable strings in the Performance Lab, Embed Optimizer, and Image Placeholders plugins were incorrectly using the 'default' text domain. I’ve updated these strings to use the correct text domain based on each plugin’s slug.

[mukeshpanchal27]

Check my comment on PR: #2061 (comment)