Preserve explicit transition duration and timing function when overriding transition property

[adamwathan]

This PR changes the behavior of the transition-{property} utilities to respect any explicit timing function or duration set by the user using the ease-* and duration-* utilities.

Say you have this HTML:

<div class="transition-colors duration-500 ease-out lg:transition-all">

Currently, the transition-duration and transition-timing-functions will be reset to their default values at the lg breakpoint even though you've provided explicit values for them.

After this PR is merged, those values will be preserved at the lg breakpoint.

This PR also adds duration-initial and ease-initial utilities to "unset" explicit duration/timing-function values so that the defaults from classes like transition-all will kick in, without having to specify their explicit values.

[RobinMalfait]

Nice! Just missing a changelog entry.