SkPlainTextEditor: from experimental to modules

Change-Id: I8896283ee3a57af926a43f6647e27059d52dd7a8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/237146
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
14 files changed