Spec: Advanced Tab Switcher (#3753)

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## Summary of the Pull Request
This is the spec for the Advanced Tab Switcher. This would allow the user to navigate through a vertical list of tabs through a UI, similar to those found in VSCode and Visual Studio.

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## References
#1502: Feature Request: Advanced Tab Switcher
#973: Ctrl+Tab toggling between two tabs

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## PR Checklist
* [x] Spec for #1502
* [x] CLA signed.

.github/actions/spell-check/dictionary/dictionary.txt

@@ -190749,6 +190749,7 @@ Imer
 Imerina
 Imeritian
 IMF
+img
 IMHO
 imi
 imid
@@ -252147,6 +252148,7 @@ MrsSmith
 MRTS
 MRU
 Mru
+mru
 M.S.
 MS
 MS.