Change Diagnostic.code to 'string | number'

I'm writing a LanguageService plugin for TypeScript that returns custom diagnostics. A `Diagnostic` needs to have a `code` property of type number. I don't have a number. Instead I can provide a string that represents the name of the rule producing the diagnostic. In addition a random number I would choose looks quite ugly in the tooltip VSCode displays for the Diagnostic. I know TypeScript only ever creates Diagnostics with numeric `code`, but it's too limiting for plugins. I looked it up and the LanguageServer protocol specifies it as `code?: number | string`. My current workaround is to use a type assertion. Until now nothing bad has happened.