Indicate submenu item level in admin nav menu editor and customizer nav menu editor - #6318

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Props rcreators, joedolson, afercia.

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@afercia All changes have been amended.

Please see inline comments

All the requested changes are pushed.

Thanks for addressing the coding standards points I raised @Rcreators
I understand part of this PR is code copied from already existing code there's some more occurrences of missing spaces between parehthesis and single quote (' or ') that would be nice to fix if possible. Thanks!
Other than that, I would leave functional and final review to @joedolson

In r58306