feat: add prefers_reduced_motion flag

[conradludgate]

Implementation of ideas discussed in #1290.

Can be enabled by setting either

• PREFERS_REDUCED_MOTION=1
• ATUIN_PREFERS_REDUCED_MOTION=1
• prefers_reduced_motion = true in client config.

The implementation uses a custom now function, which is either OffsetDateTime::now_utc, or it uses a cached time value.

Checks

• [x] I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• [x] I have checked that there are no existing pull requests for the same thing

[ellie]

Looks good! Might be worth also adding NO_MOTION? I know it's technically not "none", but just to align with NO_COLOR

Not a standard or anything like the color one, but in case it does become one

[ellie]

Thank you for this!

I wonder if we can end up with NO_MOTION implemented in a bunch of tools, like NO_COLOR