chore(exec): small API edits

[meowgorithm]

Small ergonomic edits to the long-awaited exec API. After this we should be good to do a release.

The tl;dr here is that we've added a helper command for running Exec on an exec.Cmd and, in turn, removed WrapCommand.

// Before
cmd := tea.Exec(tea.WrapCommand(c), fn)

// After
cmd := tea.ExecProcess(c, fn)

Side note: I did most of this over the weekend whilst at the dog groomer’s office.

[caarlos0]

looks neat!