Tmux sessiongroup support -- Make ZSH_TMUX_AUTOCONNECT better on multiple concurrent terminals

[kjelderg]

Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• I have read the contribution guide and followed all the instructions.
• The code follows the code style guide detailed in the wiki.
• The code is mine or it's from somewhere with an MIT-compatible license.
• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.
• If the code introduces new aliases, I provide a valid use case for all plugin users down below.

Changes:

• Added new feature to plugins/tmux when ZSH_TMUX_AUTOCONNECT is enabled. This new feature leverages tmux session groups to improve the experience when using multiple virtual terminals. Each virtual terminal is connected to a separate session in the same session group -- sharing the same windows with other terminals but allowing each session to view a different window.
• Added a new boolean variable, ZSH_TMUX_SESSIONGROUP (default: true), that selects between the new session group functionality or the previous attach-based functionality.

Other comments:

There was a related ticket (#8680) that was moved to a discussion, but I do not think made it into the project.
I have been testing this for a few months now as my daily driver. It seems vastly preferable when connecting with multiple terminals (I do one terminal on each monitor) than the existing behaviour.
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