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/interpreter/value.cpp

@@ -161,7 +161,7 @@ static auto GetNamedElement(Nonnull<Arena*> arena, Nonnull<const Value*> v,
         } else if ((*func)->declaration().is_method()) {
           // Found a method. Turn it into a bound method.
           const auto& m = cast<FunctionValue>(**func);
-          if (!m.declaration().is_virtual()) {
+          if (m.declaration().virt_override() == VirtualOverride::None) {
             return arena->New<BoundMethodValue>(&m.declaration(), me_value,
                                                 &class_type.bindings());
           }