[Spomky]

Applies the Slabs visual identity v2.0 to this repository, following page 28 of
the brand guidelines, "GitHub and Packagist".

• The project mark goes at the top of the README, in a <picture> so it
  follows the reader's theme: the colour version on light, the inverse one
  (which keeps the orange cursor) on dark. Width and height come from the
  mark's own ratio so it does not stretch.
• The title is replaced by the one-line description. The mark already
  carries the project name, so the heading was saying it twice. This is the
  header the guidelines draw: mark, one sentence, then the badges.
• .github/SECURITY_ADVISORY_TEMPLATE.md is the advisory template from
  page 49: the fields for the GitHub draft form, the description body, the
  release note and the social post.

The repository description has been set to the same sentence, so GitHub and
Packagist read alike.

Still open, and deliberately left out of this pull request because it changes
content rather than identity: page 28 also asks for four badges at most and
the sections Install, Usage, Documentation, Security, Sponsor, in that order.

The marks are served from Spomky-Labs/brand,
so a change to the identity reaches every repository at once.