[clangd] Check for editsNearCursor client capability under experimental capabilities (llvm#114699)

This is done to support clients which only support adding custom
(language-specific or server-specific) capabilities under
'experimental'.

Fixes clangd/clangd#2201

Author: HighCommander4

clang-tools-extra/clangd/Protocol.cpp

@@ -504,6 +504,16 @@ bool fromJSON(const llvm::json::Value &Params, ClientCapabilities &R,
                   P.field("offsetEncoding")))
       return false;
   }
+
+  if (auto *Experimental = O->getObject("experimental")) {
+    if (auto *TextDocument = Experimental->getObject("textDocument")) {
+      if (auto *Completion = TextDocument->getObject("completion")) {
+        if (auto EditsNearCursor = Completion->getBoolean("editsNearCursor"))
+          R.CompletionFixes |= *EditsNearCursor;
+      }
+    }
+  }
+
   return true;
 }