theme: add rectangles

[felsenhower]

This PR adds the rectangles theme which is inspired by LaTeX beamer presentations using the outer theme miniframes and inner frame rectangles.

It is somewhat similar to the university theme but designed to be a bit easier to use.

In contrast, all the meta-data like title are part of the theme initialization and new-section() is a dedicated function.

Note: There remains a bug where the first slide after a new-section() is missing the header and footer, with the exception of the first section / slide after #title-slide(). This seems rather obscure, so I'm not entirely sure if this isn't a bug in polylux' section handling?

The example in book/src/themes/gallery/rectangles.typ looks fine, but this does not:

#import "polylux.typ": *
#import themes.rectangles: *

#set text(font: "Fira Sans", weight: "regular")

#show: rectangles-theme.with(
  authors: ("Author A", "Author B"),
  title: "Title",
  subtitle: "Subtitle",
  date: "Date",
)

#title-slide()

#new-section("First section")

#slide[
  = First slide

  #lorem(20)
]

#new-section("First section")

#slide[
  = Second slide

  - Bullet points...
  - They are using the accent color!
]