Validate extensions having string value for extension kind while publishing

[sandy081]

We changed extensionKind property in package.json to be an array instead of string. Please see microsoft/vscode#85036 and microsoft/vscode#85819 for more details.

Hence we would like to have a check to validate extensions having string value and warn them while publishing.

This is not needed immediately, we want to give extension authors a month or two to adopt and then we can add this check.