Basic support for impl virtual override keyword (#2493)

Features:
* Add basic support for `impl` virtual methods (override virtual method)
* Error on invalid declaration for `impl` and `virtual`, covering simple use cases
* Add `abstract` fn parser-only support

Changes:
* Modify parser to handle new function specifiers, resolve conflicts
    * Group `virtual_override` and `FN`, and group `impl_kind` and `IMPL` to avoid ambiguities with around `impl` token parsing
* Add new `VirtualOverride` enum for function declarations, and matching `virt_override() -> VirtualOverride` getter
* Update function declaration logic

Depends on #2462

explorer/ast/declaration.cpp

@@ -342,14 +342,15 @@ auto FunctionDeclaration::Create(Nonnull<Arena*> arena,
                                  Nonnull<TuplePattern*> param_pattern,
                                  ReturnTerm return_term,
                                  std::optional<Nonnull<Block*>> body,
-                                 bool is_virtual)
+                                 VirtualOverride virt_override)
     -> ErrorOr<Nonnull<FunctionDeclaration*>> {
   DeducedParameters split_params;
   CARBON_ASSIGN_OR_RETURN(split_params,
                           SplitDeducedParameters(source_loc, deduced_params));
   return arena->New<FunctionDeclaration>(
       source_loc, name, std::move(split_params.resolved_params),
-      split_params.self_pattern, param_pattern, return_term, body, is_virtual);
+      split_params.self_pattern, param_pattern, return_term, body,
+      virt_override);
 }
 
 void CallableDeclaration::PrintDepth(int depth, llvm::raw_ostream& out) const {