generated types incompatible with exactOptionalPropertyTypes

Nobody has claimed this yet.

the generated types contain some contradictions which typescript does not detect due to the `strict` flag in the tsconfig.json being set to `false`.

this is a problem when a consuming project tries to enable `exactOptionalPropertyTypes`: now typescript considers these contradictions to be hard errors and refuses to compile.

one example are VectorTileSource and the interface ISource it implements:

ISource says "the prepare property does not have to exist, but if it exists, it has to be a function."
```
interface ISource extends Evented {
...
readonly prepare?: () => void;
...
}
```

On the other hand VectorTileSource says "the property prepage must exists, and it has to be a value of type undefined.
```
export declare class VectorTileSource extends Evented implements ISource {
...
prepare: undefined;
....
}
```

i'm not sure why VectorTileSource requires a prepare property to exist - if this is not strictly necessary, just removing the prepare property would solve the issue. May be confusing for a user who sets prepare and it unexpectedly does not do anything, though.

another way to solve this would be to change ISource do use `readonly prepare?: () => void|undefined;` which is a bit unsatisfying because now there are two different ways to express that prepare is not a function which should be called.

in general i think a typescript library should strive for the most strict typescript variant as not to limit how strict a consuming project wants to be configured.

Start with tsconfig.json and the generated declarations for ISource and VectorTileSource, focusing on the prepare property and other optional-property contradictions. Check the declarations with exactOptionalPropertyTypes enabled in a TypeScript consumer configuration; done means the generated types compile without these incompatibilities while preserving the intended API behavior.

Written by the indexing model from the issue text.