878c141adc
Adds the [DiagnosticTag][diagtag] LSP construct to the LSP support headers. I also added a unit test file to validate that the `tags` array is omitted entirely if it's empty. The LSP spec requires that `Diagnostic::tags` be an array; in order to conform to that I used `std::vector`, as `SmallVector` doesn't have JSON decoding support (you can encode it to JSON, but not decode it from JSON). [diagtag]: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#diagnosticTag