Fix strict null errors in 'vs/base/common/linkedList.ts'

[mjbvz]

Part of #60565

Tracks updating vs/base/common/linkedList.ts to have strict null checking. This file is heavily depended on and blocks enabling strictNullChecks in other files in common

Steps

1. Add ./vs/base/common/linkedList.ts to the include section of src/tsconfig.strictNullChecks.json
2. Run yarn strict-null-check -- --watch
3. Fix strict null check compiler errors
4. Verify normal build still works

[mjbvz]

Handled by 740ac11