Add async reset option for `FlipFlop`

[mkorbel1]

Motivation

In addition to #69, it would be nice to be able to use an async reset on the flop.

Desired solution

Add a way to configure the flop to reset using async reset. Perhaps a separate named argument for asyncReset?

Alternatives considered

No response

Additional details

No response

[mkorbel1]

Also add for flop

[mkorbel1]

Should have either asyncReset, reset, or no reset, but not both.

Perhaps flop(clk, a, asyncReset: reset