Insert new line at the cursor.

[mk-fan]

Issue #, if available:

• N/A

Description of changes:

• When shift + enter, insert the newline at the current cursor
• Minor refactoring

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[mk-fan]

After inserting a new line, the undo is not working and it was an existing issue. Investigating more for a workaround.

[ppiliaev]

Do we still need this pr?

[mk-fan]

Do we still need this pr?

Yeah but I'm still looking for a fix of the undo not working issue.