Update types.ts

[navanshu]

When using this package with typescript. Typescript compilers throws error that you are not using "EnumChangefreq". but as I saw the compiled output of the program doesn't really uses it. So it is better to give users the freedom and correct type both.

[huntharo]

@navanshu - Thanks for the PR! This seems reasonable. It's not a breaking change because it still works with the Enum and allows the exact same set of string values in the end.

I think we should:

1. Rebase this onto latest master branch
2. Extract the list of strings as a type like this (it might be possible to use some type magic to get the type to be created from the enum):

/**
 * @inheritdoc {EnumChangefreq}
 */
export type q = 'daily' | 'monthly' | 'always' | 'hourly' | 'weekly' | 'yearly' | 'never';