fix: AutoSuggestBox should keep focus when popup is opened

[MartinZikmund]

GitHub Issue (If applicable): closes #7675

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

AutoSuggestBox loses focus when its popup opens because IsLightDismissEnabled defaults to true on Uno instead of false as in UWP (and IsLightDismissEnabled popups may capture focus when opened)

What is the new behavior?

• Popup.IsLightDismissEnabled is now false by default to match UWP
• Added FeatureConfiguration.Popup.EnableLightDismissByDefault that allows turning the legacy behavior back on where needed
• Added tests to cover the default, and the text change behavior of AutoSuggestBox

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Validated PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
• Commits must be following the Conventional Commits specification.

[gitpod-io]