Consolidate attribute names

[elringus]

• Rename [JSImport], [JSExport] and [JSPreferences] to [Import], [Export] and [Preferences] (they're under the Bootsharp namespace, so the context is sufficient)
• Make [Import] and [Export] applicable to static methods (in addition to assemblies)
• Remove [JSInvokable], [JSFunction] and [JSEvent] (detect events from imported method name pattern)

This will streamline the API and eliminate confusion with [JSImport] and [JSExport] from System.Runtime.InteropServices.JavaScript, as well as Blazor's [JSInvokable].