Adding optional alias to plugin definitions for git clone operations

[pnavais]

Rationale
When two plugins are defined using a similar name, they will be cloned to the same location effectively overriding the plugin contents.

How this works
Adding an optional alias suffix to plugin definition to use it in git clone as output directory.
This is specially useful in case of conflicting plugins (i.e. same name) , for example, dracula/tmux, catppuccin/tmux.

set -g @plugin '<plugin>;alias=<alias>'

Using this feature we can define an alias like :

 set -g @plugin 'dracula/tmux;alias=dracula'
 set -g @plugin 'catppuccin/tmux;alias=catppuccin'

this will effectively clone both plugins using the optional alias provided.