Introduces `fill`, which fills between two `Fraction`s, use this instead
of `yHalfs` and friends wherever they're used, which also means we can
remove `rect`.

This commit does change alignment of the vertical/horizontal eighths in
certain cell sizes, but the change is for the better IMO. Also changes
the center-point alignment of smooth mosaics for odd cell widths, but
the change is no more than half a pixel at worst and is probably an
improvement ultimately.

This is a fairly large rework of how we handle the sprite font drawing.
Drawing routines are now context-less, provided only a canvas and some
metrics. There is now a separate file per unicode block / PUA area.
Sprites are now drawn on canvases with an extra quarter-cell of padding
on each edge, and automatically cropped when sent to the atlas, this
allows sprites to extend past cell boundaries which makes it possible to
have, for example, diagonal box drawing characters that connect across
cell diagonals instead of being pinched in.

Most of the sprites the code is just directly ported from the old code,
but I've rewritten a handful. Moving forward, I'd like to rewrite more
of these since the way they're currently written isn't ideal.

This rework, in addition to improving the packing efficiency of sprites
on the atlas, and allowing for out-of-cell drawing, will make it a lot
easier to add new sprites in the future, since all it takes now is to
add a single function and an import (if it's a new file).

I reworked the regression/change testing to be more robust as well, it
now covers all sprite glyphs (except non-codepoint ones) and does so at
4 different sizes. Addition/removal of glyphs will no longer create diff
noise in the generated diff image, since the position in the image of
each glyph is now fixed.

Upstream is now mostly pure Zig and the build.zig.zon.* files are
generated directly by zon2nix. The JSON file is no longer used as an
intermediate file but is retained for downstream packager usage.

This PR adds a translation for German `de_DE`.

Additionally it excludes all `*.po` files from the typos CI action.

Some comments on the decisions I made (open to discuss them):

- I choosed to use `du` instead of `Sie` as this seems appropriate
  to me.
- I added `Window` (`Fenster` in German) to all split commands
  as it appears more naturally to me.