We do unsafe things in a bunch of places

[DHowett]

originally filed by @zadjii

after merging zadjii-msft#244, we need to fix these:

• src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/CommandItemViewModel.cs : Model_PropChanged
• src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ListViewModel.cs : OnFilterUpdated
• src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ListViewModel.cs : FetchItems
• src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/MarkdownPageViewModel.cs : FetchContent
• src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/PageViewModel.cs : InitializeAsync
• src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/PageViewModel.cs : Model_PropChanged
• src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/PageViewModel.cs : ShowException

In dev/migrie/f/log-messages I have the page context hooked up to the extension context, so we can use that branch to fix this