Use abbreviation "decl" instead of "declaration" (#3382)

Part of switching to the [abbreviations we've decided to
use](https://docs.google.com/document/d/1RRYMm42osyqhI2LyjrjockYCutQ5dOf8Abu50kTrkX0/edit?resourcekey=0-kHyqOESbOHmzZphUbtLrTw#heading=h.pph7i5m5un7q).

I will rename files in a follow-up PR.

language_server/language_server.cpp

@@ -106,7 +106,7 @@ void LanguageServer::OnDocumentSymbol(
   for (const auto& node : parsed.postorder()) {
     clang::clangd::SymbolKind symbol_kind;
     switch (parsed.node_kind(node)) {
-      case Parse::NodeKind::FunctionDeclaration:
+      case Parse::NodeKind::FunctionDecl:
       case Parse::NodeKind::FunctionDefinitionStart:
         symbol_kind = clang::clangd::SymbolKind::Function;
         break;