Add simplistic editor to samples to showcase TextEditingDeltas (#1217)

* Add simplistic editor to samples

* Toggle styles fix

* update name

* Add to CI and dependabot

* update minimum dart sdk

* fix pubspec

* update README

* Update README

* Add shift + arrow key selection and copy/paste keyboard actions

* Update README.md

* copy edits to readme

includes a sentence in each layer description whose accuracy I still need to verify

* update pubspec

* update pubspec.lock

* Fix bug with disabling expansion of replacement when cursor is at edge of replacement

Co-authored-by: Renzo Olivares <roliv@google.com>
Co-authored-by: Craig Labenz <craig.labenz@gmail.com>

.github/dependabot.yaml

@@ -141,6 +141,10 @@ updates:
     directory: "simplistic_calculator/"
     schedule:
       interval: "daily"
+  - package-ecosystem: "pub"
+    directory: "simplistic_editor/"
+    schedule:
+      interval: "daily"
   - package-ecosystem: "pub"
     directory: "testing_app/"
     schedule: