Cell aware select all shortcut

**User story**
As a user, i would like the cmd/ctrl + a shortcut to be aware of the cell (block) i am in.

**Acceptance Criteria**
- when a user is in a cell and hits `cmd/ctrl` + `a` the cell should command should should select all thats in the cell and not the whole document

The issue names no file, test, or entry point. Start by locating the cell/block keyboard-shortcut handling in the JavaScript authoring UI and reproduce Cmd/Ctrl+A inside a cell. Done means the shortcut selects only the current cell's contents rather than the whole document.

Written by the indexing model from the issue text.