Fix grammar 'a' to 'an' when necessary

I see a lot of 'a' uses in comments instead of 'an'.

Ex:

• a entity
• a alt
• a author
• a image
• ...

Many modules and core files are concerned. I suggest to make all changes here instead of one issue per module since it's only minor documentation issue.

Note: .js and .css files should be generated from .es6 and .pcss.css files by run

$ cd core
$ npm run build:js
$ npm run build:css

Not touch them directly