Set --epr-sheet-x/--epr-sheet-y CSS variables

[indutny-signal]

Howdy!

I thought I upstream the patch that I ended up applying to my fork.

This commit adds --epr-sheet-x/y CSS variables to each non-native emoji. They can be used to turn emoji images into sprites through object-fit/object-position and calc. Example (with 2x scaling):

.EmojiPickerReact.epr-main  button.epr-emoji .epr-emoji-img {
   max-height: calc(2 * var(--epr-emoji-fullsize));
   max-width: calc(2 * var(--epr-emoji-fullsize));
   height: calc(2 * var(--epr-emoji-fullsize));
   width: calc(2 * var(--epr-emoji-fullsize));
   padding: calc(2* var(--epr-emoji-padding));
   object-fit: none;
   object-position: calc(-1 * var(--epr-sheet-x) * 66px) calc(-1 * var(--epr-sheet-y) * 66px);
   transform: scale(0.5);
 }