Refactoring how macOS terminal alternatives are enabled

[fcaraujo]

As stated in the contributing notes, I'm first bringing this issue up for discussion here before proposing any major refactoring.

It's my first time contributing, and I'm fixing issue #137 with PR #142. :)
As this is my first touch, I want to keep it short and simple while I familiarise myself with the project and we start shipping something working.

Let's cut to the chase: while adding this macOS feature, I noticed a couple of similarities between some implementations when enabling terminal alternatives such as alacritty, kitty, and hopefully WezTerm once the PR is merged. 🙏😅

As this list grows, they can probably use a single script passing the app's name as a parameter, and we don't repeat ourselves.
Although there are some minor details regarding the osx_enable.sh script we would need to think about the following:

• current state: the template of *.plist file might get affected since the other implementations might not have the same params, approach, etc. - ideally, we should find a way to unify and use a single one;
• docs: parts that refer to the script's name that need to be authorised;
• also, people not familiar with it could get suspicious if we chose a weird script name;
• name suggestions? Mine would simply be osx_start_tmux.sh and keep it simple, but of course we need to consider Terminal and iTerm and any other variances.

In the near future when I have some time available, I will start a draft PR proposing some stuff, linking this issue.
I am looking forward to hearing back from the maintainers. 🤙