Report workspace/triggerReindex server capability

If the server is configured to support background indexing then report
to clients that it 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

@@ -1079,6 +1079,14 @@ extension SourceKitLSPServer {
       ? nil
       : ExecuteCommandOptions(commands: builtinSwiftCommands)
 
+    var experimentalCapabilties: [String: LSPAny] = [
+      "workspace/tests": .dictionary(["version": .int(2)]),
+      "textDocument/tests": .dictionary(["version": .int(2)]),
+    ]
+    if self.options.backgroundIndexingOrDefault {
+      experimentalCapabilties["workspace/triggerReindex"] = .dictionary(["version": .int(1)])
+    }
+
     return ServerCapabilities(
       textDocumentSync: .options(
         TextDocumentSyncOptions(
@@ -1117,10 +1125,7 @@ extension SourceKitLSPServer {
       typeHierarchyProvider: .bool(true),
       semanticTokensProvider: semanticTokensOptions,
       inlayHintProvider: inlayHintOptions,
-      experimental: .dictionary([
-        "workspace/tests": .dictionary(["version": .int(2)]),
-        "textDocument/tests": .dictionary(["version": .int(2)]),
-      ])
+      experimental: .dictionary(experimentalCapabilties)
     )
   }