[structural view] add on/off preference for structural outline

[martinlippert]

We should include a preference to enable/disable the new structural outline view based on document symbols. At the moment, the structural view in the VSCode outline view can be enabled/disabled via a system property for the language server, but we should change this to allow users to experiment with this more easily.

[martinlippert]

The switch currently happens here: https://github.com/spring-projects/sts4/blob/main/headless-services/spring-boot-language-server/src/main/java/org/springframework/ide/vscode/boot/app/SpringSymbolIndex.java#L710

This should be moved to a preference instead of a check for a system property.

[martinlippert]

The default value should be "off" for this case for now, so that need to enable the new structural outline explicitly.

[BoykoAlex]

I've added the setting on the LS, and client sides (Eclipse, VSCode) d4bbdf4.