Update theme names and migrate legacy settings (#302085)

* update theme names & make experimental themes new default

* feat: migrate legacy theme settings and update notification handling

* test: add unit tests for migrateThemeSettingsId function

* feat: add command to try new default themes and update notification message

* feat: refactor tryNewDefaultThemes command to use Action2 class

* feat: update theme names to remove 'Default' prefix and improve theme selection logic

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Regenerated all 126 baseline files (108 regular + 18 tree-sitter) by running the colorize integration tests.

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Co-authored-by: mrleemurray <mrleemurray@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>