Shift + Enter Shortcut should execute cell or render markdown even if in edit mode

[claudiaregio]

There is still an influx of customer feedback where the Shift + Enter shortcut key is not behaving as expecting in our notebooks.
I believe this is due to the fact that core contributes multiple shift + enters, and their notebook one is being trumped by the other condition. Let's contribute Shift + Enter as well so it overrides and we make sure it works for our users.

**Edit: Let's make sure this applies to Ctrl + Enter as well

[joyceerhl]

Markdown cells aren't rendered for ctrl+enter when in edit mode either.

[claudiaregio]

Let's make sure that behavior persists for ctrl + enter too

[IanMatthewHuff]

Validated. Shift+Enter / Ctrl+Enter work in edit and command mode. I can look at the Ctrl+Enter => control mode when it merges.