This change adds a new configuration setting that sets the default
language mode of new untitled files created with Ctrl+N
(workbench.action.files.newUntitledFile).  This activates the user's
desired language mode when a new untitled file is created.

Fixes #8729.

Fixes #19137

- Introduced an internal boolean property 'overridable'
- Set this property to true while registering configuration
- Support inheriting this property from parent configuration node level

Turning hot exit on by default on Insiders to get testing.

Due to issues #14151, #14149 & #14182