Rename `tsType` and `tsEnumNames` to `x-tsType` and `x-tsEnumNames`

[finom]

Thank you for the great project. Would be great if these custom properties would be prefixed with x- as it's a widely used pattern that allows to extend strict schemas, such as JSON Schema or OAS. Of course, the old keys should stay there so the update doesn't break existing tools, but they should be deprecated.