Allow cmd+1..9 to select pinned tabs

[ayoy]

Task/Issue URL: https://app.asana.com/0/0/1202659052151493/f

Description:
Add TabIndex API that "converts" an integer index into TabIndex for a given TabCollectionViewModel.
Make showTab main menu action aware of pinned tabs, and use this API to select correct tab.
This patch also adds extra assertions to TabIndex.first(in:) and TabIndex.last(in:) and
fixes TabIndex.next(in:) when there are only pinned tabs.

Steps to test this PR:

1. Open multiple tabs
2. Pin a tab, Verify that cmd+1 selects the pinned tab, and cmd+2 selects the first unpinned tab
3. Pin more tabs, verify that cmd+1..9 select proper tabs
4. Have more than 9 tabs in total, verify that cmd+9 selects always the last tab
5. Have a couple of pinned tabs, close all unpinned tabs, verify that cmd+9 selects the last pinned tab
6. Unpin all tabs (you can use Debug -> Reset pinned tabs), verify that cmd+1..9 works correctly without pinned tabs.

Testing checklist:

• Test with Release configuration

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Internal references:

Software Engineering Expectations
Technical Design Template
When ready for review, remember to post the PR in MM

[tomasstrba]

LGTM! 👍