Add input dialog requested event

[CypherPotato]

Summary

• Adds InputDialogRequested for JavaScript alert, confirm, and prompt dialogs
• Adds polymorphic event args: InputDialogEventArgs, ConfirmInputDialogEventArgs, and PromptInputDialogEventArgs
• Adds dismiss, confirmation, and prompt input response APIs for handlers

Related

• Companion native callback PR: Wire input dialog callbacks photino.Native#180

Validation

• dotnet build Photino.NET.sln --no-restore
• dotnet build Photino.Test\Photino.Test.csproj --no-restore from photino.Native

Notes

• Native C++ build was not completed locally because the installed MSBuild cannot find the required v143 platform toolset.