Add .details-reset

[muan]

A follow up for #346.

@shawnbot has recommend that we bring .details-reset out into Primer soon rather than later. I have 2 PRs in github/github needing this class as well.

I put .details-reset in buttons since it's resetting <summary> and <summary> is like a button.

The summary element should be mapped to a disclosure triangle role in accessibility APIs that have such a role. ... In accessibility APIs that do not have such a fine grained role, the summary element should be mapped to a button role.

Feel free to let me know if there is a better place for them. :)

/cc @primer/ds-core

[shawnbot]

Yay, thank @muan! I think we should move the class into a new module eventually, but this makes sense to me as a first step.

[broccolini]

Merging this into v10, we'll need to remove the dupe code in the /stylesheets/components directory in https://github.com/github/github/pull/80630