feat: TypeBuilder needs stronger semantics

[sxlijin]

There are a lot of things that users can't do with TypeBuilder that they should be able to do, e.g. delete fields.

#2483 does a lot of work to implement TypeRegistry, which should address these issues.

I'm going to dupe all other TypeBuilder improvement issues against this one, so we can make sure that we address all of them before shipping #2483.

See also:

• Reorder dynamically added fields in a class #1526 Reorder dynamically added fields in a class

[sxlijin]

#2483 will address this