Add transition animation to tab width when returning to normal after mouse close

[petemill]

Fix #7001

note that this functionality will appear broken, as it does without the animation in master currently, due to a bug which is fixed by #11436.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan (manual):

flex tabs

• Make the window narrow
• Open ~7 tabs (enough for the tabs to fill the width of the container and get smaller)
• Mouse over the first tab's close button and click it
• Close another couple tabs (optional)
• Mouse away from the tabs bar
• Observe animation of tabs back to normal size

Max-width tabs

• Make the window wider
• Open ~3 tabs (not enough for the tabs to fill the width of the container and get smaller)
• Mouse over the first tab's close button and click it
• mouse away from the tabs bar
• Observe there is no animation of tabs back to normal size, since they did not change size

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[cezaraugusto]

nicely done, animation seemed a bit slow for me but definitely not a blocker and up to design-team the final decision, looks great from engineering side and this looks/feels super cool so merging ++

[luixxiul]

@petemill @cezaraugusto Is this issue related with this PR? The tab overflows instantly.

[bsclifton]

Just tried it out- this looks and feels great 😄 👍 Well done!

[petemill]

@luixxiul do you have repro steps for that? I'll check it out with these changes and before

[luixxiul]

Here is a STR:

1. git fetch upstream master && git rebase upstream/master (ie I am not quite sure if this PR is the cause of the issue)
2. npm run watch
3. npm start
4. Open 10 tabs (until the tab size changes)
5. Click the close button on a tab
6. Move the mouse cursor from the tabs bar

And the tab overflow issue above #11438 (comment) should be reproduced.