Add `Erase file contents` and `Append file contents` file options

[alexrintt]

Warning This pull request depends of #85, so it can't be merged before this one.

This PR does improvements on the file card:

Changes within /example project

• Implement functionality: create a file within a folder through Create custom document button.
• Add a button to edit the file contents (Danger button).
• Add a button to erase the file contents (Danger button).
  • The content is written by the user through a text field dialog.
• Add a button to append (FileMode.append) contents to a file (Danger button).
• Modify the current button behavior implemented on #85: now it overwrite the file with a random number to make easier to see if the file is really being overwritten.
• Add a confirmation dialog before all write based operations (delete, overwrite, append, edit), including folders.
• Add a collapsed state on the file card (it's now default) and a expanded state.
• Add icon thumbnails when image thumbnails are not available for apk image/* video/* directory/* documents.
• Refactor FileExplorerCard component (Main file component).
• Add a limit of 10mb when trying to read a file content.

Changes within the plugin

• Replace all uses of PartialDocumentFile with DocumentFile class, affected APIs: listFiles.
• All fields of the DocumentFile class it's now nullable except of Uri uri.
• In order to make DocumentFile usable within listFiles API, were added 2 new fields: id, parentUri and lastModified now returns a DateTime? instead of a Future<DateTime?>.
• createFile now doesn't requires content or bytes param, it's a empty String by default.

To see details of writeToFile API refer to #85, #79 and #61.