vim: Add gt and gT bindings for Markdown preview mode

[Pranav2612000]

What does this PR do?

• Adds default keybindings gt for navigating to the next tab and gT for navigating to the previous tab in markdown viewer mode

Why do we need this change?

• While previewing markdown files, the default vim bindings (gt and gT) do not work for navigating between tabs. These bindings work everywhere else, which provides a non-consistent experience for the user.

How do we do this change?

• Update the vim mode bindings to explicitly add handling for this mode

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[Pranav2612000]

@cla-bot check

[cla-bot]

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[ConradIrwin]

@Pranav2612000 thanks for this!

It turns out it didn't work quite right because the preview component is not an editor, so the action didn't trigger. I've fixed that and also made the binding work on all non-Editors (like screen share, and welcome screen etc.)

[zed-industries-bot]

	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

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[Pranav2612000]

Great! Thank you for the fix @ConradIrwin

It turns out it didn't work quite right because the preview component is not an editor, so the action didn't trigger.

Oh! Weird! Apologies for sending in the wrong fix. I tested out with a local build on a mac and it seemed to work. Doesn't the Markdown preview have the "MarkdownPreview" context ( atleast thats what logs told me ), and so the bindings should apply to it?

Anyways, the fix to make it work on all non-editors is amazing ❤️ ! Thank you very much 🙇🏾

[Pranav2612000]

Ahh, I see some issues on this branch. Seem to be low priority. I'll love to work on them. Let me know what you think.

1. Operations like 3gT which should go back multiple tabs doesn't work
2. gT with the menu open switches tabs ( It didn't previously )