Add 'offset' option to useScrollSpy hook

[andreaspersson-sciber]

This PR adds an 'offset' option to the useScrollSpy hook, used when calculating the active heading with respect to the top of the viewport.

• offset - offset from the top of the viewport to use when determining the active heading, by default 0 is used

This allows the hook to accommodate pages with floating headers or other elements that obscures the top of the viewport.

Example usage:

function Demo() {
  return (
    <TableOfContents
      scrollSpyOptions={{
        offset: 70 // Offsets the "top of the viewport" used when calculating the active element
      }}
    />
  );
}

This fixes an issue where the Table of Contents may display an unexpected heading as active when using the AppShell header.
https://codesandbox.io/p/sandbox/mantine-react-template-forked-wlrnk8

Without offset:

("Title 3" is the topmost visible title in the user's window, but TableOfContents shows "Title 2" as active)

With offset:

TableOfContents shows the expected Title 3 as active, accommodating for the header.

[rtivital]

Thanks!