Hide underscored attributes by default, give option to unhide

[MahdiBM]

Is your feature request related to a problem? Please describe.

Describe the solution you'd like
@_rawLayout and @_staticExclusiveOnly should be hidden by default.

Describe alternatives you've considered
Current behavior is fine by me but I don't think it's the best behavior for the average user.

Additional context

[award999]

Would need support from LSP: swiftlang/sourcekit-lsp#2213