refactor: remove padding and add full-header, full-footer to support footnote

[OrangeX4]

(Original page layout ↑)

To support footnote, we need to modify the page layout of the slide from:

page(
  header: header,
  pad(
    body
  ),
  footer: footer,
)

to

set page(
  header: negative-pad(header),
  footer: negative-pad(footer),
)
body

structure.

Where let negative-pad = pad.with(x: -margin.x), which extends the header/footer to occupy the entire width of the page.

As a result, we remove the self.padding parameter and add self.full-header and self.full-footer to control whether this feature is enabled (not enabled for the simple theme, but enabled for all other themes).

This PR is a breaking change, but it enables support for footnote #26 and allows normal usage of set align(horizon) set rule.

cc @sefidel and @fran-miao

[OrangeX4]

[sefidel]

LGTM, tested functionality on simple theme.