fix(help): add styled whitespace with configurable background

[rrrodzilla]

Added WhitespaceStyle fields to both short and full help Styles struct, llowing customization of whitespace appearance between keys and descriptions. Previously whitespace was unstyled, now it matches the aesthetic of other elements and can be customized (e.g. with background colors for testing).

• Added ShortWhitespace and FullWhitespace to Styles struct
• Modified ShortHelpView to use styled whitespace between key and desc
• Modified FullHelpView to use styled whitespace between key and desc
• Added tests to verify whitespace styling behavior

Fixes #571

[adamgoose]

Looks like this whitespace between the truncated keys and the ellipsis remains unstyled.

https://github.com/charmbracelet/bubbles/blob/a2a300a0065e3f212a59db1bb77b493ee2854aef/help/help.go#L228