Rename --transition-timing-function-* variables to --ease-*

[adamwathan]

This PR renames the --transition-timing-function-* theme variables to --ease-* to more closely match the utility names and be a bit more terse in general.

  @theme {
-   --transition-timing-function-in: cubic-bezier(0.4, 0, 1, 1);
-   --transition-timing-function-out: cubic-bezier(0, 0, 0.2, 1);
-   --transition-timing-function-in-out: cubic-bezier(0.4, 0, 0.2, 1);

+   --ease-in: cubic-bezier(0.4, 0, 1, 1);
+   --ease-out: cubic-bezier(0, 0, 0.2, 1);
+   --ease-in-out: cubic-bezier(0.4, 0, 0.2, 1);
  }

This is part of a bigger set of changes where we're renaming other theme variables as well with the same goals, since many existing theme variables like --shadow-* and --radius-* are already not using the explicit CSS property name.