Report workspace/triggerReindex server capability

Report to clients that the server supports the
`workspace/triggerReindex` request. This lets clients expose this
functionality only if SourceKit-LSP is configured to support it.

Author: plemarquand

Sources/SourceKitLSP/SourceKitLSPServer.swift

@@ -1104,6 +1104,7 @@ extension SourceKitLSPServer {
       experimental: .dictionary([
         "workspace/tests": .dictionary(["version": .int(2)]),
         "textDocument/tests": .dictionary(["version": .int(2)]),
+        "workspace/triggerReindex": .dictionary(["version": .int(1)]),
       ])
     )
   }