Move die helper materials to dice/ subdirectory

This declutters the toplevel a bit.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/die-cooldown-extractor b/dice/cooldown-template-extractor
similarity index 100%
rename from die-cooldown-extractor
rename to dice/cooldown-template-extractor

diff --git a/die.svg b/dice/die.svg
similarity index 100%
rename from die.svg
rename to dice/die.svg