Typing Form Submission Values with tiny-invariant

Our title, slug, and markdown currently have a TypeScript warning on them because they can be of the value FormDataEntryValue or null.

To handle this error, we are going to be using invariant from tiny-invariant to throw warnings if the data being passed isn't a string.

We are also going to type our errors down in our JSX so they aren't just of type any.

To do this, we type the errors variable as ActionData which will take the title, slug, and markdown as types.