[#2190] Fix VaButtonDropdown dropdown appearance and alignment for split and left-icon

[RVitaly1978]

Description

close #2190

changes:

• set prevent-overflow to VaDropdown
• set vertical-align: middle
• update examples

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

[m0ksem]

@RVitaly1978, why do we need setting prevent-overflow on dropdown? I think we must proxy dropdown props to components like VaButtonDropdown and VaSelect. By default, VaButtonDropdown shouldn't have prevent-overflow, but user must be able to set it.

If it doesn't work because of missing prevent-overflow, please, open another issue and merge this PR. Otherwise, proxy VaDropdown props like we do with VaInput in VaDateInput.