vdiffr 0.4.0

This is a github-only release that is meant to help you migrate from
vdiffr 0.3.x to vdiffr 1.0.0. The 1.0 release of vdiffr includes two
major changes. It switches to testthat 3e for snapshot management and
it uses an updated SVG engine for the generation of snapshots. The
github-only 0.4.0 release only includes the new SVG engine to make it
easy to compare cases with manage_cases().

This intermediate step in the migration of snapshots is optional. You
can also choose to update directly with the 1.0.0 release but you
won't be able to compare the new snapshots to your old ones.

Note that smaller points are expected in the new snapshots because of
a bugfix.

• Following an update to the graphics engine of R 4.1.0 that causes
  spurious failures due to subtle changes in graphics generation,
  vdiffr snapshots are now skipped on old versions of R. The plots are
  still generated on all versions so any errors during plot drawing
  will be detected by testthat.

• The SVG generation engine of vdiffr has been updated (@thomasp85).
  Font sizes are now precomputed for the first 50000 unicode points.
  This allows deterministic computation of text box extents without
  the freetype and harfbuzz dependencies.

  Note: The main visible change of this update is that points now look
  smaller. Points generated with the previous SVG engine were too large.

• vdiffr is now licensed as MIT (#95).