Restructuring the tmux package to use a central struct for command execution to help with testability.
To limit the scope of the this PR it includes the use of a package level instance of the new tmux.Command struct and an init function to initialize it. Once all of the functions using the tmux cli are updated to use the Command struct directly that code can be removed in favor of a single tmux.Command instance configured in the cli.
Restructuring the tmux package to use a central struct for command execution to help with testability.
To limit the scope of the this PR it includes the use of a package level instance of the new tmux.Command struct and an init function to initialize it. Once all of the functions using the tmux cli are updated to use the Command struct directly that code can be removed in favor of a single tmux.Command instance configured in the cli.
Tested locally and ran unit tests.