Add NestedMenuCandidate concept item

[NotWoods]

Broken out from the main menu2 PR. Adds a new concept menu type and corresponding view holder.

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Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry or does not need one
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

After merge

• Milestone: Make sure issues closed by this pull request are added to the milestone of the version currently in development.
• Breaking Changes: If this is a breaking change, please push a draft PR on Reference Browser to address the breaking issues.

[codecov]

Codecov Report

Merging #7467 into master will decrease coverage by 0.00%.
The diff coverage is 38.88%.

@@             Coverage Diff              @@
##             master    #7467      +/-   ##
============================================
- Coverage     77.58%   77.58%   -0.01%     
+ Complexity     5127     4999     -128     
============================================
  Files           688      683       -5     
  Lines         24841    24551     -290     
  Branches       3667     3608      -59     
============================================
- Hits          19274    19047     -227     
+ Misses         4057     4037      -20     
+ Partials       1510     1467      -43     

Impacted Files	Coverage Δ	Complexity Δ
...lla/components/browser/menu/ext/BrowserMenuItem.kt	66.66% <0.00%> (-6.07%)	0.00 <0.00> (ø)
...onents/browser/menu2/adapter/LastItemViewHolder.kt	80.00% <0.00%> (-20.00%)	2.00 <0.00> (ø)
.../browser/menu2/adapter/MenuCandidateListAdapter.kt	29.72% <0.00%> (-3.61%)	7.00 <0.00> (ø)
...ser/menu2/adapter/NestedMenuCandidateViewHolder.kt	0.00% <0.00%> (ø)	0.00 <0.00> (?)
.../components/browser/menu/item/BackPressMenuItem.kt	100.00% <100.00%> (ø)	5.00 <1.00> (+1.00)
.../browser/menu/item/BrowserMenuHighlightableItem.kt	91.48% <100.00%> (ø)	20.00 <3.00> (ø)
...mponents/browser/menu/item/BrowserMenuImageText.kt	100.00% <100.00%> (ø)	9.00 <3.00> (ø)
...ponents/browser/menu/item/ParentBrowserMenuItem.kt	95.00% <100.00%> (-2.37%)	13.00 <1.00> (-1.00)
...components/support/sync/telemetry/SyncTelemetry.kt	85.71% <0.00%> (-1.39%)	41.00% <0.00%> (ø%)
...onents/lib/crash/notification/CrashNotification.kt	94.11% <0.00%> (-0.17%)	2.00% <0.00%> (ø%)
... and 12 more

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[psymoon]

Looks good to me!

[NotWoods]

bors r=psymoon

[bors]

Build succeeded:

• complete-push