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/testdata/class/virtual_method.carbon

@@ -25,8 +25,7 @@ base class C {
 }
 
 class D extends C {
-  // TODO: This should instead use `impl` when supported
-  virtual fn Foo[self: Self]() -> i32 {
+  impl fn Foo[self: Self]() -> i32 {
     return 3;
   }
   fn Bar[self: Self]() -> i32 {