Feature: new builtin picker tabpages

[quantumsnowball]

Description

This PR suggests to add a builtin picker tabpages. This picker is similar to the buffers picker, but its focus is on tabpages and windows instead. When a neovim user is having many split windows in different tabpages, switching between them using the buffers picker is usually difficult. Instead, by using the new tabpages picker, user should be able to switch to the specific window on a specific tabpage. I found this picker very useful when using neovim as an IDE, where different tabpages are usually holding different file types (e.g. js, css, json files on separate tabpages). In this case, switching by the buffers picker will mess up the workspace, and tabpages picker can speed up file switching and preserving the workspace layout.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list relevant details about your configuration

• Tested on multiple split windows over multiple tabpages
• Tested on Manjaro Linux and MacOS

Configuration:

• Neovim version (nvim --version): v0.8.0-dev-nightly-6-gbb7853a62
• Operating system and version: Manjaro Ruah 21.3.7 / MacOS 12.5

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)

[kuator]

Looking forward to this one!

[hinell]

It's unthinkable for me to not have tab-switcher. It's definitely nice to have a bult-in one, but there is a well maintained extension:

• https://github.com/LukasPietzschmann/telescope-tabs

[jamestrew]

I hate to nullify any efforts but I don't think we'd be merging this.

There's already been discussions on telescope having too many builtin pickers, many of which are seldom used by the maintainers or users in general, and the idea of splitting up all the pickers into group as their own plugins been brought up. #1228

Especially if there's already a well maintained extension, I don't think there's a need to absorb this tab related picker into telescope core.

[hinell]

@jamestrew I would rather a have tabswitcher than a buffer switcher actually. It would be the most useful switcher ever.

@quantumsnowball I would suggest to move this PR into a stand-alone plugin for Telescope.