[TECHNICAL] Kotlinize PreviewTextFragment

[joragua]

Related Issues

App: https://github.com/owncloud/android/issues/4356

• [x] Add changelog files for the fixed issues in folder changelog/unreleased. More info here
• [x] Add feature to Release Notes in ReleaseNotesViewModel.kt creating a new ReleaseNote() with String resources (if required)

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QA

QA checks https://github.com/owncloud/android/pull/4376#issuecomment-2076712173

[jesmrec]

Regression over text preview

• [X] Preview .txt file from file list (both orientations)
• [X] Preview .txt file from details view (both orientations)
• [X] Preview markdown file (both orientations)
• [X] Preview markdown file in text mode (both orientations)

Not much more to test here, text previews are OK and now in Kotlin. Move on!