Add suport for tmux-mode-indicator

[rbpatt2019]

Adds support for tmux-mode-indicator.
Classically, this plugin changes its background colour by mode. There is no single
bg setting in the plugin, but rather a separate one for each mode. To make the
catppuccin bg change dynamically as well, we would then need to re-implement the
logic for detecting the mode. To avoid this complexity, this PR sets all the
bg colours to the catppuccin background colour and only changes the text.

[rbpatt2019]

Close #236.

[vdbe]

Only change segments related to this PR (### Mode Indicator module)

[vdbe]

Some options to present the modes with colors:

• change to fg color to thm_blue/yellow/red/cyan/... (if this remains readable)

• create the equivalent of https://github.com/MunifTanjim/tmux-mode-indicator/blob/7027903adca37c54cb8f5fa99fc113b11c23c2c4/mode_indicator.tmux#L46 but just for background color and use that as color (don't think you need a rewrite to make it dynamic misread original PR)

• Same as above but set $thm_bg should only effect this module (don't really like this)

[vdbe]

Feels like it would make more sense to extend the session module with color indicators for copy and sync, but then you lose the text representation.

tmux/status/session.sh

Line 6 in e7cd05f

color=$(get_tmux_option "@catppuccin_session_color" "#{?client_prefix,$thm_red,$thm_green}")

Starting to lean more to making this a builtin module instead of requiring a plugin if extending session is not enough, since re-implementing in an opinionated way would be as much or less work as configuring it in an opinionated way.

What do you think?