Improve the select_tab switcher with context and [New tab] option

[chriszarate]

The PR improves the usability of the select_tab switcher to provide additional context and functionality. This makes it more useful as a kind of "on-demand" tab bar.

Lately I've been using tab_bar_style hidden for the simplicity and focus that it provides. In this mode, the select_tab command becomes very useful as a way to navigate tabs. Specifically, I use it to:

1. Switch to an existing tab that I recently used.
2. Get a quick awareness of what tabs I have open and decide whether to reuse a tab or open a new one.

The current select_tab implementation works well for the first use case but not the second. Personally, I want the ability to go to the select_tab screen and decide what to do next. This PR:

• Adds a "[New tab]" option, which is helpful in situations where an existing tab cannot be reused.
• Includes the current tab and marks it with "[current tab]".
• Marks tabs with multiple windows with the window count ("[3 windows]").

I find this gives me a much better jumping off point:

This is definitely a personal viewpoint, so I don't expect this to be merged as-is. I opened a PR because it seemed more illustrative than an issue. I'm happy to help iterate on this if there are ideas about how to improve it.

It's also worth noting that none of this strictly requires a change to kitty and is easily implementable as a kitten. I've done so here:

https://github.com/chriszarate/dotfiles/blob/daf4c3036e91f76e88c0f63f64b7c285e7029e68/kitty/kittens/select_tab.py

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I should point out that, as the name implies, the most important job of select_tab is to select the other tabs.

• The first option cannot be New tab.
• It doesn't make sense for the current tab to appear in the available options, but it can be displayed as text at the top. However, if you are already in the current tab, it is still redundant to show it again.

I understand your desire for a consistent presentation (in a list that is all executable next actions) and appreciate the inspiration, but this is, as you say, a personal preference.

[kovidgoyal]

I think having number of windows is a good idea have implemented it. As for showing the current tab, also no harm, as a subtitle.