Clear editor highlighting on reload

[yusufm]

Summary

• clear existing editor highlighting after replacing text during reload from disk
• refresh clear-text attributes before scheduling the new highlight pass
• add a lifecycle regression for the external reload path

Related to #378.

Validation

• xcodebuild test -workspace 'MacDown 3000.xcworkspace' -scheme MacDown -destination 'platform=macOS' -only-testing:MacDownTests/MPDocumentLifecycleTests/testExistingDocumentClearsHighlightingBeforeReloadHighlight -only-testing:MacDownTests/MPDocumentLifecycleTests/testExistingDocumentTriggersRenderOnReload -only-testing:MacDownTests/MPDocumentLifecycleTests/testExistingDocumentTriggersHighlightOnReload

Note: the last only-testing selector is not present on main, so Xcode ran the two matching lifecycle tests.