Explorer: support .base to initialize parent class from struct (#2361)

Relates to https://github.com/carbon-language/carbon-lang/issues/1881

- Add support for `.base` field in structs for [parent class initialization](https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/classes.md#constructors)
    - Disabling base class initialization without `.base`
- Support class constructors (`Create() -> Self`) for base classes
- Direct access to base class(es) attributes with `object.var` remains unaffected

Changes:
- Add `TypeChecker::FieldTypesWithBase` to help assessing if a struct with `base` fields can be converted to a class
- Add a new `base_type()` attribute+getter to `NominalClassDeclaration` to as a first step to allow resolving parametrized classes
- Add a new `base` attribute+getter to `NominalClassValue` that contains the base class `NominalClassValue`. It is currently used mainly to get and set members of a class object.
- Add `Interpreter::ConvertClassWithBase` to build `NominalClassValue` from a init struct, that contains `.base` fields with either `NominalClassValue` or `StructValue`
- Add `FindClassField` to find a field in a class or its base classes
- Remove superfluous `ClassDeclaration::base()` in favor of `ClassDeclaration::base_type()`

Limitations;
- Though some work is done in that direction, parametrized base class where time is not know at the declaration site are not supported. Namely the example below does not compile
```
base class A(T:! Type) {}
class B(T:! Type) extends A(T) {}
```
But this one is functional already
```
base class A(T:! Type) {}
class B extends A(i32) {}
```

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

explorer/testdata/class/class_inheritance_function_call.carbon

@@ -33,7 +33,7 @@ fn Main() -> i32 {
   // Note: D.FunctionC(0); is not accessible
 
   // Calling function from class object
-  var d: D = {};
+  var d: D = {.base={}};
   d.FunctionC(1);
   d.FunctionD(2);
   return 0;