Declare PaddingValues and Dp types to squelch warnings

[marcprux]

Fixes warnings introduced in #336:

skip-ui/Sources/SkipUI/SkipUI/Layout/Presentation.swift:84:34 Skip is unable to determine the type of this expression. Consider declaring the variable type explicitly, i.e. 'var v: <Type> = ...'
skip-ui/Sources/SkipUI/SkipUI/Layout/Presentation.swift:85:32 Skip is unable to determine the type of this expression. Consider declaring the variable type explicitly, i.e. 'var v: <Type> = ...'
skip-ui/Sources/SkipUI/SkipUI/Layout/Presentation.swift:86:33 Skip is unable to determine the type of this expression. Consider declaring the variable type explicitly, i.e. 'var v: <Type> = ...'
skip-ui/Sources/SkipUI/SkipUI/Layout/Presentation.swift:87:32 Skip is unable to determine the type of this expression. Consider declaring the variable type explicitly, i.e. 'var v: <Type> = ...'
skip-ui/Sources/SkipUI/SkipUI/Layout/Presentation.swift:88:40 Skip is unable to determine the type of this expression. Consider declaring the variable type explicitly, i.e. 'var v: <Type> = ...'
skip-ui/Sources/SkipUI/SkipUI/Layout/Presentation.swift:89:42 Skip is unable to determine the type of this expression. Consider declaring the variable type explicitly, i.e. 'var v: <Type> = ...'
skip-ui/Sources/SkipUI/SkipUI/Layout/Presentation.swift:90:39 Skip is unable to determine the type of this expression. Consider declaring the variable type explicitly, i.e. 'var v: <Type> = ...'
skip-ui/Sources/SkipUI/SkipUI/Layout/Presentation.swift:91:39 Skip is unable to determine the type of this expression. Consider declaring the variable type explicitly, i.e. 'var v: <Type> = ...'
skip-ui/Sources/SkipUI/SkipUI/Layout/Presentation.swift:822:1 This extension will be moved into its extended type definition when translated to Kotlin. It will not be able to access this file's private types or fileprivate members