fix bug introduced by dependabot #205

[kortina]

The dependabot update #205 updated a few libraries which caused the following errors:

 [email protected] compile
> ./node_modules/.bin/tsc -p ./

src/BacklinksTreeDataProvider.ts(148,7): error TS2611: 'command' is defined as a property in class 'TreeItem', but is overridden here in 'BacklinkItem' as an accessor.
src/BacklinksTreeDataProvider.ts(164,7): error TS2611: 'tooltip' is defined as a property in class 'TreeItem', but is overridden here in 'BacklinkItem' as an accessor.
src/BacklinksTreeDataProvider.ts(174,7): error TS2611: 'description' is defined as a property in class 'TreeItem', but is overridden here in 'BacklinkItem' as an accessor.
src/BacklinksTreeDataProvider.ts(193,7): error TS2611: 'iconPath' is defined as a property in class 'TreeItem', but is overridden here in 'BacklinkItem' as an accessor.

I resolved these by changing the command, tooltip, description, and iconPath accessors to be private functions which I then use to initialize the corresponding properties in the class constructor method.

I did a run with debug mode on and verified I was still seeing backlinks show up in the left sidebar: