Allow tabs for indentation

[christian-konrad]

Is your feature request related to a problem? Please describe

Currently, in the FEEL Expression Editor, tabs are not supported for indentation, as the tab is reserved in embedding components (such as the properties panel).

Describe the solution you'd like

Indentation with tabs should work with another shortcut, like ctrl+tab / cmd+tab.

Describe alternatives you've considered

Force users to indent with single spaces, which will end up in chaos.

Additional context

[marstamm]

As discussed with @christian-konrad , we want to focus on #13 first to reduce the amount of times manuall tabbing is required.

Tabbing inside a web-app is used for navigation, capturing tabs in an in-line editor might lead to accessibility problems. The best solution would be to offer a pop-out editor that then acts as a full editor.