Ability to remap keyboard shortcuts

[microbit-matt-hillsdon]

Is your feature request related to a problem? Please describe.
Background: microsoft/pxt#10464 is upgrading to the latest Blockly keyboard navigation and #6152 will add a keyboard controls documentation in the app.

For some users the default keyboard shortcuts are not practical. One common scenario is potential conflicts with screenreader keyboard shortcuts, which are often customised. For this reason to improve accessibility we should allow the user to change the keys.

This should apply to:

• Blockly shortcut registry keyboard shortcuts
• Global shortcuts, e.g. those proposed on Add global shortcuts to improve keyboard navigation #6151
• Potentially Monaco/JavaScript Editor, though this has an overwhelming number of shortcuts and commands that could potentially have shortcuts, so it might be best tackled separately or with a UI approach that allows blocks-focussed users to easily ignore them.

Describe the solution you'd like
There's lots of examples of this type of UI in similar apps.