[SAGECell] Code Completion

[rctheriot]

SAGEcell support for code completion.

[renambot]

Initial overview:

• can be enabled now for js/ts/...
• UI issue where code completion is not displayed in the right place (since we use zoom, translation, ...)
• how to add python code completion: monaco exposes some API for that...

[renambot]

• String completion in inline mode inside cell now (tab to complete)
• In the drawer, I enabled the menu suggestions
• All this is just text completion, based the text in the input element.