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.

toolchain/parse/tree.cpp

@@ -29,7 +29,7 @@ auto Tree::Parse(Lex::TokenizedBuffer& tokens, DiagnosticConsumer& consumer,
   context.AddLeafNode(NodeKind::FileStart,
                       context.ConsumeChecked(Lex::TokenKind::StartOfFile));
 
-  context.PushState(State::DeclarationScopeLoop);
+  context.PushState(State::DeclScopeLoop);
 
   while (!context.state_stack().empty()) {
     // clang warns on unhandled enum values; clang-tidy is incorrect here.