Fixed a bug when comments are allowed but trailing commas aren't

[abc-55]

This was probably a typo. This change doesn't affect VSCode because in VSCode, either both comments and trailing commas are allowed, or both are not allowed.

[abc-55]

I have just discovered that this would change the behavior of Monaco:

• Monaco sets allowComments to true (https://github.com/Microsoft/monaco-json/blob/6653d96ddd9d5659147b5a096daf292685d1e66f/src/monaco.contribution.ts#L46)
• Monaco doesn't provide documentSettings (https://github.com/Microsoft/monaco-json/blob/39e2f4e7093adebbd6ba5861aa524c97d38012c5/src/jsonWorker.ts#L68)

This means that trailingCommaSeverity is DiagnosticSeverity.Error and commentSeverity is undefined
https://github.com/Microsoft/vscode-json-languageservice/blob/7b0bc0dcf45ab6411429e986716f969f92773850/src/services/jsonValidation.ts#L52-L53

However, because of the bug, trailing commas were ignored too.

The question is: What is the right behavior? Should allowComments also affect trailing commas?