SP Formatter VSCode extension creates a bridge between VSCode and SP Formatter web extension, makes it available to modify your Column Formatting JSON inside VSCode and have it synchronized to browser with live preview.
Then your .json file will be the subject for additional intellisense.
Additional intellisense includes:
Fluent UI icon names suggestion
rel, role, target property values
full CSS styles intellisense with documentation and values based on CSS standards
To make additional intellisense to work, you should remove default $schema property from your JSON formatting file. If you still need it and at the same time want to leverage additional intellisense, you can setup a custom gulp task, which generates files with $schema property, like in this sample gist
npm run watch
Open Chrome or Edge with SP Formatter and enable it for the current tab.
VSCode extension should have connected to the web extension.