Enable json language support for code-snippets files

[RedCMD]

Re-enables the json-language-features extension on snippet files
after it was broken with #189176

Fixes #203206

uses the new jsonLanguageParticipants feature from #198583

"main": "" is required because "extensionDependencies": [ "vscode.json-language-features" ] does not start the json-language-features extension without it

"activationEvents": [] is there because when publishing an extension it is required when "main" is present
Manifest needs the 'activationEvents' property, given it has a 'main' property.

[aeschli]

Sorry, only now I realize that snippets was added as new built-in language.

json is a basic language extension and must not depend on the json-language-features which is a rich language extension.
They typically run on different extension hosts. The basic language extension is purely declarative and can run on the UI extension host also on the Web

The proper setup would be to create a rich extension snippet-language-features to do that activation. But we don't want to go there for the built-in extensions.

For now, I suggest adding it as a known language to the json-language server and not yet use the jsonLanguageParticipants contribution point.

Was snippets the main motivation for the PR that added the jsonLanguageParticipants ?

[RedCMD]

The main motivation was for my own personal extension
however I knew it could have been a possible fix for this issue