Update samples to use repo analysis options, Fix sample templates and a ton of analyzer issues (#77868)

Text editing shortcuts involving the arrow keys are no longer handled by RenderEditable's RawKeyboardListener, they use the new Shortcuts setup.  First PR in a plan to port all text editing keyboard handling to shortcuts.

Very simple right-click context menu for Windows and Linux in a Material-esque style.

A very minimal right-click menu for Mac desktop.

Adds MediaQuery.maybeOf to replace calling MediaQuery.of(context, nullOk: true), and removes the nullOk parameter. Also changes MediaQuery.of to return a non-nullable value, and removes many instances of the ! operator, reducing the possible places where a null dereference could occur.

Enables custom text selection menus by allowing selectionControls to be passed to TextField et. al.

* migrate material to nullsafety

* address review comments

* address review comments

* fix build

Simple docs fix for characters.

Complete the migration to the TextSelectionTheme by deprecating the opt-in flag and text selection properties in ThemeData. (#66482)

Turn the opt-in default for ThemeData.useTextSelectionTheme to true so that everything uses the new TextSelectionTheme by default.