Re-export types from ast submodule

[matklad]

Right now, AST types aren't actually accessible to consumers of djot, as they aren't re-exported from the ast.ts file, and only index is visible downstream.

I had to update typescript as export type * seems to be a fairly new thing.

[matklad]

I guess we can as well just manually list all the types here?

[matklad]

reverted typescript update, explicit listing seems more readable for consumer.

[matklad]

@jgm could you take a look here? I am trying to use djot in a new context, and inability to name ast types makes writing overrides somewhat painful.