SplitButton: Fixing some styling bugs related to the divider

[khmakoto]

Pull request checklist

• Addresses an existing issue: Fixes #0000
• Include a change request file using $ npm run change

Description of changes

While writing VR tests and testing Button perf I realized that the styling for the SplitButton divider was wrong:

However, this did not repro on the component example page. After investigating it became apparent that the use of Stack in the example code was masking this issue. This PR fixes that styling issue so that the divider now looks correctly both in the component example page and in its independent use:

Once this PR merges, this will also serve to unblock #9159, which was not being merged because SplitButton vr tests didn't look right.

Finally, this PR also adds display: inline-flex to the root of both MenuButton and SplitButton, so that they render inline like HTML buttons do (this issue was highlighted by perf-tests that rendered a large number of these Buttons).

Focus areas to test

(optional)

Microsoft Reviewers: Open in CodeFlow

[msft-github-bot]

Component perf results:

Scenario	Target branch avg total (ms)	PR avg total (ms)	Target branch avg per item (ms)	PR avg per item (ms)	Is significant change	Is regression
PrimaryButton	93.228	97.974	0.932	0.980	false	false
BaseButton	36.997	38.420	0.370	0.384	false	false
NewButton	66.961	84.906	0.670	0.849	true	true
button	6.440	6.675	0.064	0.067	false	false
DetailsRows without styles	197.389	195.046	1.974	1.950	false	false
DetailsRows	192.727	194.772	1.927	1.948	false	false
Toggles	42.330	42.600	0.423	0.426	false	false
NewToggle	73.069	69.960	0.731	0.700	false	false
DocumentCardTitle with truncation	25.894	25.386	0.259	0.254	false	false

[msft-github-bot]

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[khmakoto]

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